The cheaper alternative would be to buy one that has already been converted, or buy a factory turbo car. The GC/GF WRX's, early GD/GG WRX's, GT Foresters and even some XT Foresters are sub $10k these days!
However, I'm going to answer your questions with more questions...
What is your budget?
Do you have much mechanical knowledge yourself?
What are you wanting out of the car? Ie, Dyno king? Track warrior? Street cred? Fast daily?
The swap has been done many, many times before ...and on various budgets.
If you're starting with nothing apart from your existing shell, you'd probably spend the better part of $10k with mostly DIY work. Add labour to that if you don't want to do it yourself and I imagine you wouldn't get much change from $15k - depending how many issues you run into along the way.
There are stacks of threads in the Builds and Projects section which would be well worth checking out!
All the best with it all. We're here to help you through it!