No doubt turbos can be more labor to install. Cost is roughly a third more than, say, a good centrifugal unit (all but the PowerCharger/ATI sc's will require taping oil out of the pan, just like turbos). Power is about double, and lots of the extra cost are for things like tubular k members, headers, and coil overs, which have a value in and of themselves. Custom fabbing is pretty much a thing of the past - modern kits have all the pieces, its mainly a bolt on affair, though lots MORE bolts than the typical supercharger. You might check out www.modularpowerhouse.com
. They list a very complete Twin Turbo install at $9300. A similar VorTech install (with intercooler, headers, tubular K member, tubular A members, etc.) would run about $6400. A 2.2 Kenne Bell will set you back about $7200 installed with comparable parts list.
Most centrifugal units really need an intercooler to work best, and a gear swap to get them into their sweet spot on the power curve. Superchargers in general also put added stress on the crank, take horsepower from the motor to create boost, and centrifubal superchargers have their own lag problems which are very similar to properly sized turbochargers.
You avoid lag problems with either an eaton or twin screw (kenne bell) positive displacement blower, but they cost almost as much as the turbos (though they are a much easier install). Great low end torque - not near the top end horsepower of the turbos. Some companys, by the way, offer single turbo kits that are closer in price to the Keene Bell items.
Strut braces (I have a steeda item on my GT) can be clearance problems for superchargers and turbos, though some quick measurements before installation might save some problems later.
If you go with a supercharger or turbo, you will likely have to remove the CAI and see what it will bring second hand. Your pulleys will be ok with the turbo, NOT with any of the superchargers. Your tune will need updating, but there's a good chance that you can get a new download for your diablo rather than have to buy another one.
Assuming you have the normal space under your hood as a 99-04, you have plenty of space for whatever you want to do - the gap between the waterpump and the radiator can hold virtually any centrifugal sc or turbo you might want to run. Some kits require relocating the batter to the trunk (again, a move that has advantages of its own), though many do not. Some folks are using the space gained from doing this to install methanol injection (or coolers, depending on application) that helps lower intake temps and add power (Anderson Ford makes a good one).
Unfortunately, staying n.a. with these cars is just about as expensive as installing a power-adder, without the fun factor and with less power gain.
If you've driven an 03-04 terminator and fallen in love with the low end torque, consider a Kenne Bell (1.7 unless you are really ambitious, then check out the 2.2). No lag time, no waiting.
For a long time, that was my personal plan - until I drove a car with the twin turbo rig. 700 rwhp will make you change your religion.