As in the kind with two roller things that crush stuff. I would guess about 8d6 per round, and probably require a strength check to escape.
Depending on the specifics, I'd say 6/8/10d6/8/10 would seem right.
It's with hazards like this that we really need to emphasize all that the hit point value is an abstraction of. As funny as the image of a high-level dwarven fighter bouncing and rolling in one of these and somehow not dying may be.
The hardness of stone is 8, and it has 15 hit points per inch of thickness. That's more for walls, though, so I'd give a 6" boulder, say, 40 hit points? Most rocks that big might have to bounce around in there a bit, so at 8d6, a minimum damage roll (8) would do nothing but a maximum (48) would destroy it in one shot, so 8d6 sounds about right.
Escaping would probably be Strength or Escape Artist. You may want to check out some of the mechanical traps that already exist for inspiration, particularly the Crushing Cage trap.
I'd recommend working out the CR for the encounter first, and basing it on that.
The rules for custom traps allow you to set a CR based on average damage (usually +1 per 10 hp average damage) so use that as a guide for how dangerous the rock crusher should be compared to your group's APL.
Considering APL isn't a bad bit of advice, though I'd suggest that particularly low-level PC's just wouldn't be able to survive something like this.
Escaping could involve a strength check, Acrobatics or a grapple of some sort.
If you want to see a rock-crusher death-trap with a detailed write-up, check out
In this adventure, the party is tricked by an NPC into entering the empty hopper through a secret door. They get locked in, then a confederate of that NPC begins operating the crusher. The PCs have a round (or maybe two) to react before the floor drops out beneath them, dumping them into the grinding mechanism. It's a really nasty death-trap for the adventure's level (3rd) but there are a number of things that the PCs can do to increase their chances of surviving.
When I updated the adventure from 3.0 to 3.5, I edited the trap description to make each step clearer. I also expended the section addressing quick-thinking PC tactics, and broke it out into a new sidebar that I titled "No, Mr. Bard, I Expect You to Die."
Sometimes I think Pathfinder RPG could use a Chunky Salsa Rule.
Then I slap myself and go back to playing awesome.