The last gallon will cost nothing but a single bullet. That is, if we haven't found an alternative resource. But like others said, this isn't something to kill yourself over right now.
Doesn't gasoline evaporate in the air? Eventually it'll all be gaseous. I don't see how this is such a crazy smart idea. Again, the real money in petroleum isn't in asphalt grease, motor oil, candle was, etc. It's in the gasoline. The rest is just by-products that we found uses for.
to quote the episode of futurama "A Big Piece Of Garbage" "Fry: who cares, it'll be thousands of years before that happens! Proffesor: you're right, it's none of our concerns!"