Rough prices as I've not looked at dev hardware in a while..
GameCube dev kits come in a few varieties:
GBOX is a PC looking thing with GC controller ports. It's not going to be worth it unless if he has all the connecting cards that need to go in a accompanying PC (and even then that PC would need to run WinXP or older OS with SDK software). These aren't really worth buying.
$50-500 depending on what it comes with
NPDP Reader (Red Cube with Cart slot on the top), there isn't too much you can do with it, even if it comes with a cart.
$250 or $400 if it comes with a cart with something actually on it.
NR Reader (Green cube), as above, this one will only run NR discs which are beta/prototypes
TDEV (Brown cube), as above, this one will only run NR discs but has a whopping 48MB of primary RAM and a USB port. It will require software for both the cube and the PC it connects to.
The upper prices are if the units are boxed/mint condition - since a lot of this stuff was used by testers it's usually very bad condition.