They are a herbivore, pound for pound they presumably a lot cheaper to feed then a carnivore or omnivore I'd think, even accounting for elephant's long maturation period and I gather crappy eating efficiency, which I assume the mammoth shares, I wouldn't think they'd be that expensive to feed, probably not that much worse then a polar bear or tiger. I'd even go so far as to guess one could profitably raise some for a niche meat market, akin to lobster. If their calorie of food per pound of meat is even in the same general ballpark as a cow or pig, even if still a lot higher, you could keep a pretty decent if small market for 'mammoth burgers'.

Besides, if it did work, one hardly would need to keep them around, just grab some DNA and put them back on ice for resurrection every couple centuries, and liquid nitrogen is very, very cheap, you could keep hundreds of dna samples from thousand of creatures in a closet sized freezer for a few hundred bucks a year. In this sort of context 'extinct' starts becoming a very hazy term. If they don't prove to be a generally net positive draw for zoos, to the point you can even have half a dozen small herds around the world, you just let them go extinct again until there's a good market for them again and clone yourself up some more.