I think your idea of supplying map objects for use in VTT's would be extremely popular, there are many, MANY collections of tokens for sale (the pc's and npc images, most that are pay are top down 3d renders) but there are very few collections of straight objects... i know i like my maps to look uniform, and its very difficult to get a few images made from one source never mind enough to run campaigns... and i don't like throwing in different artists objects because in most cases, different styles = different look and feel... and makes they seem out of place.
As for pricing, I don't think its all that unrealistic... your not making commercial adventures and your using a free VTT client, so the main reason to pay for it is your creativity and artwork... token packs from four ugly monsters and such go for about $4 a pack, and there are a number of images in a pack... I think its very reasonable, and as long as your getting it out there you'll get some sales, and it will also add to your experience so you can end up making much more complex, involved adventures that you can charge more for.