If you have the time, Pull-A-Part is by far the cheapest route. They do nickel and dime you on the accessories though..... You would probably be sub$1,000 for motor and trans there but you get to do the work....
There's a guy up near me that parts out a lot of GM stuff. Will Stiles on FB Marketplace. He's got a truck he's parting right now with a 5.3 for $850 which is about the going rate from what i've seen.