You can get out your trusty calculator and check Jegs or Summits web site to add up the cost to buy those parts new today (Without the shipping costs on each individual parts).
If the 12 bolt rear end is not assembled include the price of new gaskets, bearings and seals (small parts kit) plus at least $250 in labor to assemble it. I say at least as a competent mechanic with experience in assembling Spicer rear ends will probably charge you more.
Getting the rear end work guaranteed is also very important as a howler not only ruins your new gear set and new bearings but the rear must be disassembled and cleaned to get the metal shavings from the ruined gear out of the housing.
Once you have the new parts prices (it will probably stagger you to learn at how much that pile of parts costs to buy new) I would consider offering him 60% to see if he rises to the bait. This assumes he bough this stuff new. If he bought it as pile of parts from a previous hobbiest and is throwing in the towel because of some reason then he got a discount up front and you get to discount his dioscounted price. The parts prices get discounted further every time it changes hands up until the time those parts go on sale on Flea-Bay at which point the price goes back up to suggested retail price plus 10% handling fee for the seller having posted an ad to tempt you to buy a pig-in-a-poke.