about how much would it cost to turn the exterior of a regular 2006 WRX (no spoiler)..to an STI

why not try your local subaru dealer?

It would be less expensive and less hassle to just trade it on a ST l model. Why spend the money. 227 hp is more than enough. l think the handling is better on the WRX. l used to work for Subaru dealers before my accident. Great cars!
If you just want to add decals that say ST l and new wheels contact a Subaru parts department.

The best way to convert the outside of a wrx to an sti would be to go on ebay.com and search for an ST l trunk with spoiler, this will assure you that the spoiler wont leak due to it being a factory install. You also want to upgrade the hood the scoop is a little bigger on the sti. Also the ST l has HID headlights and the WRX does not you can either get the xenon blue bulbs or convert your headlights to HID. All this will run you about $900.

You should buy the WRX , if you modify it, it would be even more powerful and it is cheaper.