It can be done cheaper, but it depends a lot on the condition of those turbo's. It also depends on how much boost you're planning on running. I've seen guys turbo stock engines, the compression ratio works out well for it and the bottom end of the small blocks can handle some pretty high hp even in stock form. But if you get your air/fuel just a little wrong, run lean, and detonate those stock pistons will
disappear. You'll also need new exhaust manifolds, all kinds of new plumbing, a modified carb for blow through, upgraded fuel pump, blow off valve, etc. Even if you can get good turbo's on the cheap, there's still a lot of expensive work to be done.
Here's a link to guy over on A-bodies that used a Holset T70 (HUGE turbo) to turbo a stock 1974 318 with 185k miles on it
. Hasn't come up with dyno #'s yet, but he's run as much as 12 psi of boost through that otherwise bone stock 318. Pretty innovative use of shorty headers and turbo location too. End results are on page 7, then it gets into body work. Does some pretty cool burnouts with it! Now, in theory anyway, at 14.7 psi you should have doubled your HP, so that stock 318 should be making close to 300 hp (150 hp was a low estimate and its running a 4 barrel now...)
http://www.forabodiesonly.com/mopar/showthread.php?t=49548The real question is, how long do you think that engine will last like that? Even if you assume that 318 puts out 300hp with the turbo, that's attainable with a good modern rebuild and a naturally aspirated 318. So, if you spend enough money setting up the turbo, you get to a point where you could have had a nice reliable, naturally aspirated engine that will last a long time, vs a turbo'd stocker that won't have that kind of longevity at the same power level.