RTV = scrap value, I agree.
Price is a severely regional thing. In CA I can get cores all day long at $100-150 as they are plentiful. $75 for tail shift as they are typically roached. On the east coast, prices vary based on planetary allignment, zip code, political leaning, tea leaves, and often chicken bones. North and south are the big determinant. Many/most of the FL cores I have seen were shit with roached internals and salt damage. But they were cheap most of the time

Cores from the rust belt tend to be corroded pretty bad and cheap, but good ones command more money with people trying to get $300-400 for a side shifter of unknown provenance.
Basically, get what you can/pay what you are willing. In the end, you will be in a small ball park of cost unless someone really screws you over.