I collect those!
I have that pinky purple sparkle pony with aqua hair both moc and loose. If I were selling them I would ask $15-20 for the moc and $5-ish for the loose one.
I agree they are slow sellers. But there must be some collectors out there because I see them go really high on ebay sometimes.
The ebay prices vary a lot though, for example one single pony sold for $18.95 recently, while a lot with 8 sold for $12.
Most loose ones, if they sell at all, seem to go for $7-12.
You can look at sold and ended auctions on ebay, search for "Magic Meadow Ponies".