Price for a turbo grafx cd add on

I live down the street from a guy who buy and sells odds and ends. Anyway, hes had this turbo grafx cd add on sitting there for awhile with no price. Its in the original (huge) box and lets just assume it works. How much should I offer the guy for this? I checked ebay to get an idea of what they are worth but I couldnt find any being sold.
Edited: I just realized this isn't the Neo-Geo forums
I would see if he'll take $50 for it. If he does, thats a steal. I found mine at a flea market, with the carrying case and system card and only paid $20 for it. I had to buy the power cable, and send it in for repairs, but its worked like a charm ever since. I figure I paid around $50 for mine after all that.
uhh, $50!

I'm not familar with the price of this add-on, but I know 50 sounds way to steep...... for an odds&ends store.

Chances are this guy has no idea what he has or he would price it. He's probably waiting for someone that knows about it to price it. I would start very low. Don't let him know you are too interested, and start out at $5. I did this at an odds&ends store and got a 15" NEC MultiSync XV15+ computer monitor for $5! and it's only 5 years old! The guy was an ex-NFL player though, so money wasn't a matter.

If $5 sounds to ridiculous, try at least $10 or $20. Try to negotiate and then let it sit for another day or two. Then go back and offer what you want. He'll proably let it go for cheap then.