I've spent way too much time down this rabbit hole. The ones that always come up for sheer prize money are the big international contests—think 'International Book Award' run by various festivals, or those big standalone ones like the 'Miami Book Fair' competitions. But honestly, a lot of the absolute highest cash prizes aren't for unpublished novels; they're career achievement awards for already-published books, like the 'International Dublin Literary Award' which is like €100k but that's for a book that's already out in the world.
If we're talking about pure manuscript contests for unpublished or self-published work, the numbers shrink fast. The 'Amazon Storyteller' award had a massive purse, but it's been inconsistent. The 'Masters Review' novel excerpt contest has decent prizes, and some genre-specific ones like the 'Writer's Digest' annual competition offer good money in their novel categories, but it's often in the $5k-$25k range, not life-changing sums. A weird thing I've noticed is that some of the biggest cash prizes come with the most strings attached—exclusive submission windows, insanely specific genre requirements, or clauses about film rights.
My take? Don't chase the biggest dollar figure alone. A $2k prize from a contest that has a real track record of getting winners agent attention and publication deals is worth ten times a $10k prize from a contest nobody in publishing has heard of. I'd trade a slightly smaller check for a legit imprint's first-look deal any day. The money's nice, but the real prize is the career launchpad.