We could start a new thread just for this discussion, but the average prices they have been going for lately, i.e. market value, not retail (and charitable sellers notwithstanding) are these:
Solid colors $80-110+
Tri-colors $120-150+
Rare/limited $200-250+
That's what you could expect to pay when buying them at auction or from a seller who paid those prices for them.
If you are patient, people sometimes give them away for free, or sell them at cost. Or sell them at a personal loss, to try and do something nice for someone.