You can get a new 4 bolt main block from GM for $700. If you are a manufacturer who buys in volume, you could probably get them as cheaply as $450-$500 a piece. Not much more than a normal core and considerably less if it doesn't need much or any machining at all. So, $1500 is a reasonable price for a fully assembled motor using a new block. That is what you'll pay if you guy the new GM motor and include shipping. Now, some of these builders offer better warranties than GM, even on their reman motors.