It's cheaper than either cigarettes or alcohol, especially in places where it's available "medicinally", like SF.
Yes, I've been to those places because THC suppressed epileptic seizures I used to have (but only for several hours while it was in effect- it's an extremely expensive anticonvulsant and insurance doesn't cover it). Legal pot is particularly expensive. On what basis is pot cheaper that liquor or alcohol? Per "dose" (whatever that is for any of those three)? Cigarettes and liquor never tempted me to cut up my ATM card.
Plus, you can't survive on a 100% Taco Bell diet... just eating the pot would be more nutritious.
He: Let's end it all, bequeathin' our brains to science. She: What?!? Science got enough trouble with their OWN brains. -- Walt Kelly