As VTC said it cost alot.
The $1500 kits on Ebay are not complete. Also I spent that much just on tuning to pass emmisions test. The cost will keep on adding up. Most people shouldn't waste their money on turbos, I did but thats just my hobby car I have no plans on selling, its all for fun, Your car will always be off the road too so if you do go through with it make sure you have a spare car.
Basically what I am saying is turbo is not for everyone and the money poured in will never end, unless you can fix problems that arise yourself you better have money for the mechanics.

Also you say you want little gains but when bigger gains are just around the corner, you will always want them.

Don't mean to scare you but it is true.

Like what Robert from Hannys says, "If you want more power, buy a NSX"