The turbo itself can be had fairly cheap, if you shop well enough. The problem is, you are likely to need a new fuel pump, fuel injectors, all the hoses and fittings, a method of controlling fuel pressure, exhaust modifications, intercooler (optional), intake piping, colder plugs, something to integrate your current computer to accept the new boost levels OR a new computer all together... it's a big list.
Though, this is assuming you want to do it "right". By "right", I mean keep it reliable and able to take a decent amount of boost (when needed). There might be quite a bit you can skip if the ultimate goal is only 3-5psi.
I have a 89 turbo van. I bought the van for $400. I have well over $3K into it, just to make it reliably fast. But if I get 20MPG+ out of it, I probably didn't have fun on that last tank...
Not trying to rain on anybodies parade, I just hate to see somebody try and invest in a turbo setup, only to find that it won't ever pay for itself... versus putting the money towards something else that will.