I could do it for $1500, but I've done a few of these swaps now... and that's just the engine/transmission. Most guys also improve suspension, brakes, wheels, etc. when starting from scratch. Unless you have experience with these engines and engine swaps in general you will be exchanging time to keep the budget low. The first swap I started took me about 8 years. I helped with a few others along the way and couldn't help but tackle other aspects of building the truck at the same time, but if I had thrown more money at it, it would have taken less time.
Biggest task is sorting out the wiring harness on most swaps and if buying a pre made plug and play harness that's easily $1000 or more to the budget.
If you aren't able to weld or fabricate then you need to buy specialty swap headers or manifolds and pay someone to make your exhaust.
The phrase "while you're in there" will also be expensive. I'm not saying it is a bad phrase, but when going for budget you'll have to avoid saying it more often than not.