Core engines aren't worth much money since so many of them turn out to be worthless. I used to buy core engines all the time and I would have to scrap about 1 in 4 block. Once you get them all pulled apart, cleaned, magged and sonic checked you'll see what I mean.
It takes a lot of work to pull them all apart and clean them. And it cost money to get them hot tanked, magged and sonic checked. Just because of that I'm usually not interested in any big block core where the guy wants more than a few hundred bucks. This summer I bought 3 big blocks for $300 and it wasn't a great deal. One block was worthless, one didn't have main caps and one block appears to be okay. So after spending $300 plus a full day of labor and $200 for cleaning, magging and sonic checking I have a block that is worth $500.