Depends on many things. Look around on some completed unmentionable auction website listings for HP-15C, find 3 or 5 similar condition ones with manual if possible, average them, and then subtract off $25- for the fees and such you would pay to ebay and paypal and the convenience factor for you.
Alternatively, average them without a manual and charge $30- for the manual if spiral bound great condition, less if in worse condition or not spiral bound.
For a friend, I'd say, for one in good condition, generally around $175.
Great condition, add $100-$150.
Edited: 3 Mar 2008, 5:17 p.m.