I'll have to disagree with a couple of your points/concepts.
I think the better way to go about this is to figure out your end state/goal you wish to attain.
Will it be raced? Is fuel economy/daily drivability a concern? Do you need stock type durabilty/reliabilty?
When it comes to cost (depending on the answers to the above questions) you can spend equal amounts of money on any of the power adders. If you want to just add a quick 50 hp that you can turn on and off for weekend/racing use then by all means go with nitrous, and you'll more than likely still see 150K+ mile. If you want a bigger shot and you want the car to last more than say 50k mi post-modification; you might wanna consider beefing up the internals, or at least a tear down/inspect/rebuild.
For SC vs TC there really is no contest the turbo smacks down the SC every time with better reliability/power, yet many people think an SC is "cheaper". You can turbo/sc a stock engine just don't expect to run over 250fwhp and see 100K + miles. You'll need to beef up the bottom end to keep it reliable.
For bang for the buck power NA really does have nod. You can get a full-bore-fully-built-race cylinder head for less than $2000 including cams! dial it back a little bit we're takling less than $1500 for KILLER street cyl head. For NA power its all about the "special sauce" in the cyl head/intake mani combo. FYI you can totaly drop an SC on a full tilt NA motor it's just all about the tune.
OK done rambling
