I feed live personally, but from what I've seen of the prices, f/t is actually considerably cheaper if you're buying in bulk. If feeding live is strictly for monetary reasons, you're better sticking with f/t. Also, you'd want live RATS, not live MICE, if your snakes are big enough to eat rats.
What are the f/t you're buying? Do they come in a bag? How many to a bag? How much is it? If you figure it out to cost per rat, you'll likely find that f/t rats are cheaper than live rats (again, rats not mice). If you're buying them individually, consider buying them in bulk bags. It's much cheaper.