Cheapest form of racing will be a "spec" series. It limits the amount you can do to a car. It should require everyone run at the same weight, with the same power,etc etc. SCCA offers a couple spec classes, NASA might as well.
Figure out your potential budget. Everything.. from car to buy (if not your 911).. motor repair costs (what happens when you blow a motor.. are you sidelined a few weeks or a season), regular maintenance (oil changes, valve adjustments), brake costs (rotors, pads, calipers), safety equipment, hotel rates, entrance fees, gas, insurance, sit down and realize this will cost serious money. Heck, if you have been doing any "DE" events, you can use that for a guide for some of these costs.
Then look at the rules for the group your want to run with. If its PCA, with your current 911, you will be in a very competitive class (east coast) and therefore a potentially "expensive" class to be a front runner. Car prep is everything. Hoosiers don't grow on trees
Talk to folks who run in the class you want to run in. They will offer advice to you as long as you aren't beating them
. They can give you a very good picture of the $$ involved.
Porsches are expensive. imho, if you can't afford to "write the car off" the car is too expensive to be raced with.