either way works. but the planted tank does help keep them from picking on eachother if you have a small shoal.
average price is usually $7-8/ea. the places that claim to have them for cheaper online usually are "out of stock", or want a lot for s&h. youre better off getting them from your LFS, and if they dont have them (probably because theyre out of season), you can ask them to order them for you whenever they get them on the order sheet.