I've searched the web (you probaby have, too) and used 4301 prices, when you can find them, are all over the board. Cores are worth about $150 at most rebuilders, but this is not a core and worth much more. I've found new 4301's listed for about $975. I think most, like you and I, really have no idea what your mag is actually worth, especially with about 156 hours on it since inspection. 500 hour inspections are about $375-$400 on 4301's.
I'm guessing on value, no matter what price range I choose.
How about this: Core charge + 500 hour inspection - accumulated time. $150 + $400 - ($1.25/hr x 156hrs = $195) = $355 MINIMUM price, no impulse coupling or harness (and Eric pays shipping). Is that fair?
Doesn't really matter, as long as you are BOTH happy with the deal.