Obviously they can vary, but a general rule of thumb is that retail pricing starts at about $10/seed for one.  Most resellers offer 3-pack and 5-pack (and more), and of course the more you buy the cheaper they get.  Many of the resellers (aka Seed Banks) will also toss you a freebie of something new they're trying to push, so often you can net out at $5 or so per seed by buying a few, getting a freebie here and there, and so forth. (Greybeard Seeds often kicks in a freebie of something new they're trying to promote, for example).