Think $4 a gallon gas is already too much? Then brace yourself, says one expert.

"The prices that we're paying at the pump today are, I think, going to be 'the good old days,' because others who watch this very closely forecast that we're going to be hitting $12 and $15 a gallon, and then, after that, when world oil production goes into decline, we're going to talk about rationing," Robert Hirsch, Management Information Services Senior Energy Advisor, said on CNBC's "Squawk Box