The price of unobtainium is the equivalent of $5/gal.

By itself, this is an irrational assumption. Surely there is a supply curve for unobtainium, and $5/gal-of-gas-equivalent is one point on that curve. Now what is the supply elasticity of unobtainium? The answer to "What if we had ... " depends on a lot of extra uncontrained assumptions and could be anything.

If instead of unobtainium, we speculate about synthetic gasoline that is made from biomass via the Fischer-Tropsch Process, I think the scenario is something like this: The current situation is that synthetic gasoline can not be made at a price that competes with real petroleum based gasoline. Over time scarcity rent on petroleum will rise. Eventually, it will make sense for some FT plants to bring synthetic gasoline to market. The synthetic gasoline will gain market share and the scarcity rent portion of real gasoline price will decrease until there is a stable balance or until there starts to be a scarcity rent on the feed stock for synthetic gasoline. The market share for real and synthetic gasoline will be driven by the relative scarcity of the two different feed stocks: biomass and petroleum. The world will be such a different place from what it is now that I think it is useless to speculate on this market share.

This kind of speculation can be extended to a second substitute for gasoline. It will enter the marketplace when scarcity rents on real and synthetic from FT allow a profit from making it at market price. And also for a forth kind, and a fifth kind, etc. This progression of substitutes must end before the Earth becomes so overloaded that it can no longer support life.