I've never seen them all sold together and condition makes a huge difference, even if it's just a smell. Are they the windy wing set? If it's the windy wing releases, flurry $65, cool breeze $70, moon jumper $50, starry wings $40, sun glider $40, and whirly $30, in absolute perfect out of box condition, U.S. releases. $350-400 for the set. These are on the highest end, so for mint ponies. A broken wing can halve the price. Also check out bluerose's guide.
If you choose to sell, I think your best luck would be to sell individually. Most collectors are more likely to buy one at a time, or only need one or two for their collections.