I guess it depends on, if it's the puck mouse or Pro Mouse, and the older debut iMac keyboard or the newer "Pro Keyboard" full size one.
The Pro items have managed to hold their value on eBay, where a set can sell for almost $20 + ship. So how about $15 + ship?
For the older type iMac keyboard and puck mouse, maybe $12 + ship, choose your color.