I think the price range now is right, but there are some difficulties that are a problem when it comes to selecting the small $5.00 brushables. 
From a market standpoint, $5-6 is a perfect price for impulse buy.  That's the actual name for the stuff hanging by the registers, on endcaps or around that price point.  It's less than 1 hour worth of work at minimum wage, but still turns a profit.  I don't think they're going to adjust quality because it'd require different materials, and different production methods, meaning more expense.  They tried to do a collector-style DJ and it wasn't well-received.  Parents aren't likely going to pay more than $6 for an impulse, even if collectors would.    

The trouble ponies are having is more related to the stagnation of variety.  Hasbro finally understood this in some perspective, because they've (finally) come out with the positional ones at a slightly higher price.  They have a packaging fail though.  Small- is good!  It means more product in the same space.  More visibility = more sales.  What is not working for collectors and is also hard on stores is the open hang tab.  Try to look through a hangar of small brushables and they fall all over the place.  The cards get bent, meaning collectors don't want them (unless it's shortpacked like Holly Dash) and stores get stuck with them.  Zoning is a nightmare- most of our local Walmarts have taken to tossing overflow in wire baskets now.  Unseen at best, rummaged through and damaged more often than not.  Displaying in a collection is a nightmare.  What they need are plastic bubbles underneath the pony to allow it to stand upright on its card on a shelf, and also an open tab with a bit more plastic on one side that flexes to keep the carded figure secure on its hang peg.  Either would make the figures more visible, displayable, and easier for employees to maintain too.     

Now for design of the actual ponies:
No matter how many garish designs they plaster across the same core group of ponies, there are only so many Pinkie Pies or Rainbow Dashes that any parent or collector will buy without a significant change.  All these years and the brushables are still in the exact same pose.  Molds/ moulds are extremely expensive to make for casting.  (I know because my dad has been in casting for over 40 years.)  Even so, this line has one stock pose for the $5.00 range.  Every other line had variety of pose.  It adds to imagination and play value.  Yes, willing to pay a few dollars more for the shine brights, the tiny breezies, the posables, etc, but the line would really benefit from 1) a few new 4" brushable poses, or 2) for money-saving value on molds, reuse and development of more male brushable characters off of the shining armor mold.  Better believe THAT would get all sorts of fan interest in a hurry.

Lastly, separate pony accessories.  Not meaning that the ponies themselves couldn't be sold with their standard comb, but rather if they want to sell a lot of product, a $30.00-$75.00 playset is not going to do it.  Small play packs- kind of like ponywear used to be- but more like what would be seen in the cartoon should be made available.  G3 had a great idea with the plastic ones because they held up well and didn't need live people to sew them.
$10.00-$15.00 would be a good price point for these.  By way of idea, Pinkie Pie's candy shop would be a start.  How about a little play pack with a gumball machine, lollipops, cakes, a tiny cart to attach to the pony?  For Applejack, it's been ages since the barn set was released, and I still wish it had been a full barn instead of just the front. 
How about a small pack with some jumping fences, barrels for barrel racing, a lasso, curry brush, Big Mac?  They were on the right track with the carriages and cars, but there really is so much more they could do to add variety to the line without it becoming gimmicky.  Remote controlled cars?- yes fun, but expensive and molds need to be used a couple times or more to be worth tooling.  Massive, huge, talking Twilight Sparkle that didn't really look like her toon self?  It didn't sell well here.  Big, expensive... not really attractive exactly.  Storytellers were neat and affordable.  Good choice there for variety imo.

Anyway, different characters and more small play material would be of more interest and turn a better profit for their investment.  Hasbro really should look at the line more from a play-oriented perspective.  Even $6.00 would be manageable, but less flooding stores with the core group.  Last year was like 3.5 all over again.       

You might try Lysol antibacterial kitchen cleaner.  (Comes in a purple bottle.)  When I taught kindergarten, it removed a lot of marker types, and I always cleaned my ponies with it.  It's gentle and works well on many types of marks and will not damage symbols.  Worth a try anyway! 

Do the moonwalk baby!  Love when they put those references in!
Nice catches.   

Love everyone's stories!  Family really enhances or spoils the fun in collecting. Anything becomes more enjoyable when the folks around you support it.

I'm really lucky to have a family like this.  Mom eeked out money here and there to buy ponies for me when I was small, and still occasionally does.  When we moved into the new house, she made curtains for all my windows out of pony fabric.  She and dad bought anti-UV film on rolls for the windows.  When I was small, Dad made me a structure that took up a whole wall in the bedroom to put the ponies on.  In the new house, he put up a ton of shelving across every wall, and he actually designed the new house addition to account for the size of the collection.  Then there are the cases. Dad, my big brother, my husband and a few terrific friends have helped bring 11 breakfronts/china hutches and 4 curio cabinets upstairs.  The last two just went in this past weekend.  Then... my gem of a husband!  Ponies are frequent surprises now.  He's from Northern Ireland and my in-laws often save ponies and merchandise from there for me, but when Christmas or birthday rolls around, each time I wake up to piles of pony things from Keith.  Every so often, he'll bring me home some of the gorgeously detailed G4 plush ponies from Oly Factory.  He 'gets it' though, because he collects Star Wars and Dr. Who, and we both love the original Transformers and collect them too.  My kitties are also very supportive, especially of the new cabinets...     

I use page protectors for storing mine.   They won't damage your cards, clip into a binder for easy viewing from both sides and there are many types available inexpensively at the big box stores.  Even with sticky on them, once they are in the plastic sleeve, they are still removable.

They do look great together!  Congrats on such a nice set!

Oh well done Ivy!  That is really the hardest one to get!  I'm jealous of your new snowbabies ones.  Not much of a snowbaby fan, but they look lovely with the others!