Probably something around $10-20 depending on where you go.

It sounds like an easy fix though. Take off the tire, tube, and rim tape. Find the nipple, and thread the spoke back into the nipple. Tighten the nipple a bit and your wheel may possible get trued. If not, you could try to true the wheel yourself.

Be careful not to strip the spoke nipple.

You may also have to replace the nipple or spoke depending on if one is broken.