You can buy the stock block, short block for around $5k, with a forged crank, and roller cam.
Can save close to $1000, by going with a flat tappet cam.
http://lenwilliamsautomachine.com/455_Short_Block.html
http://www.sandovalperformance.com/engines
https://butlerperformance.com/i-2445...tegory:1234783
The Spotts site shows an aftermarket 505 short block, for $6190. Don't know if that price is current or not. Forged crank will probably add $400.
http://www.spottsperformance.com/IA2%20engine%20block2
Most say that it costs at least $1000 to do a set of unported iron heads. Alum heads are what, at least $2500, or more.
http://www.sdperformance.com/viewPro...?productID=583
http://www.krepower.com/Pontiac%20Al...er%20Heads.htm
https://butlerperformance.com/i-3024...tegory:1287735
http://www.krepower.com/Pontiac%20Al...er%20Heads.htm
In order to build from scratch & and buy all new parts, I'd say it would be hard to build a 10.50 engine for less than $8k. But, if you have ready to run heads, intake, carb, and any other necessary parts, it would obviously reduce the cost.