First of all your car is not registered right? Why would you worry about putting 93 octane in it? Not rational
As far as 3 grand for the HG, check out my HG thread, I have a break down of every part needed to replace the head gasket and it comes nowhere close to even $1,000. And thats with $200 in head work done by a shop.
If the head gasket is gonna go, its gonna go. Running 87 octane won't make it go.