Ballpark figure. A number that is “approximately correct.” As a verb, to ballpark something is “to estimate (something) roughly or casually.”
Back-of-the-envelope. A calculation “done quickly to provide a rough estimate.”
Long shot. In betting, “a bet in which the chances of winning are slight but the possible winnings great.”
Safe bet. “A thing very likely to happen.”
Bet the farm on it. This is said “to show confidence in an anticipated outcome.”