Every time someone asks for G3 prices I throw up a "NOT MATURED. THE CHEESE IS STILL AGING" disclaimer.
That said, I think $10 is a good starting point for the more common ones. If you have Birthflower/Birthstone ponies you can charge at least $30... MiB prices are all over the place really because there are so many of them out there.
That wishing well one I think will run higher. It's a bit hard to make out the ponies.
(Pm me if you have Sunny Daze)
Pretty much what @twishy said. Christmas/holiday ponies will go for slightly more, as will jewel, long haired and sets. You have an early release of Rainbow Dash which tends to go for more than Tink-a-tink-a-too or Sew-and-so . Most of those I'd sell for $10-$15, especially when factoring in shipping.