This is how my Megan and Sundance army started
I wanted enough to model the wear and then just couldnt stop buying them
As you mention some of the sets are harder to find than others and some of the individual items are tough to track down. The 2 you mention County Jamboree and Picnic in the Park turn up the most often of them all. Then Ice Princess and Flower Darling. And the harder to find sets are Sweet Dreams and By the Sea. However Sweet Dreams or Flower Darling is not as popular as Ice Princess so although harder to find its often not as expensive. For all of the sets - bar Ice Princess and By the Sea of course - the shoes are the toughest items to find. They tend to run really expensive when they come up individually or in sets and are often missing from pony wear sets unfortunately. Also hard to find are the sunglasses from By the Sea and the flower garland head dress from Flower Daring.
It may sound odd but sometimes these sets run the same price MOC as they do loose and complete, they seem to show up MOC as often as you will get a complete [with sticker, ribbon and shoes] set loose.
Assuming you have shoes but missing ribbons and stickers:
Country Jamboree - $12
Picnic in the Park - $15
Ice Princess - $25
Flower Darling - $25
By the Sea - $30
Sweet Dreams - $15