According to some quick research, the "Woolly Mammoth" (Mammuthus primigenius), is just about the size of an adult elephant. Elephants apparently eat about 300-400 lbs of vegetation/day. Let's just run with 400 to account for any increased requirements a Woolly Mammoth would have from living in an arctic environment.

The price of middling-quality hay in Kansas (where hay is relatively plentiful) is about $110/T. so it'll last you about 3 days for one adult mammoth. So you're going to need about 10 tons of hay/month, 120 tons for a year...

That's about $13,200 dollars/year for feed alone for maintinance. It wouldn't suprise me if you actually needed to feed it more, due to higher requirements and/or boredom. When herbivoires are bored, they eat, as they're designed to be eating all damned day anyway. $13,200 is a good starting point, but I definitely feel like it's the low end of our estimate. If you have to add concentrate on top of that, or any supplements (PS. you probably will), it'll become MORE expensive.

And that's the adult. The young is probably going to cost MORE, really, because it's going to have to consume milk (and god knows if whatever milk we can give it will be an appropriate formula!) at a high volume.

I do think that carnivore diets end up being more expensive than this per year, BUT, a mammoth is probably going to be ill all the gorram time because it's not adapted to any microbial habitat a zoo can possibly recreate, and cloned animals generally aren't quite as healthy as naturally produced animals. Then there's the specialized caretaking, cost of the size of the habitat, etc...etc....

Anyway. So yah. I don't think it's at all economically efficient to keep a mammoth in a zoo. At least we know how to take care of elephants...