how much does a rebuilt title due to hitting a deer affect asking price?

I would say it reduces the value at least 1/3. I would not buy one so be careful. Check with your insurance company to see it they will cover it BEFORE you buy it.

May be a bargain, but ONLY for a "permanent" subsequent owner. You MUST get it thoroughly evaluated by a BODY SHOP (NOT just a mechanic) to gauge the level of unibody damage. Imprezas are not as strong as Legacy-OBs, so it's important to see if a front rail have been crushed (as designed). If so, I'd walk away. Although it's unlikely that the shock towers were bent, the fact that such a high-value newer vehicle now has a salvage title indicates that the repair cost exceeded 2/3-5/8 of its NADA retail value. That's a LOT! Generally a deer-hit will take out a fender, hood, maybe a windshield, and the front radiator support. That's a few grand, but nowhere near "totaling" a '15 CT. Sniff much more deeply here, as a deer ain't a moose!