I've run some numbers using the forums Eladrin gave. Here's a very quick rule of thumb you can use when planning gearsets.

Not going to go into the maths here, but you can highlight the black paragraph below for info on my method.

[warning: calculus]Basically I'm considering the number of 1 damage hits required to kill a player (assuming no rounding, so with 1 PRR each hit does 0.9935 damage) as a two-variable function F of PRR and HP, and differentiating this function with respect to PRR and HP (to get dF/dPRR and dF/dHP) and looking at the ratio of these two at various plausible values. I'm then finding a linear approximation to segments of the function Y=(dF/dPRR)/(dF/dHP) to make guidelines that can be easily understood.

[/calculus]

600hp character(what I predict will be the standard on a 'squishy' melee, e.g. Rogue, at 25)

Your first 70 points of PRR are worth 3.5hp each. Your next 70 points are worth 2.5hp each, and after that additional PRR is worth 1.5 hp each.

So for raw survivability, a 600hp 50 PRR rogue should be very close to a 635hp 40 PRR rogue. And a 600hp 150 PRR rogue will be close to a 640hp 130 PRR rogue.

800hp character

As above, but add 1hp in 'value' to each PRR.

1200hp tank

The first 100 points of PRR are worth about 6.5hp each. From 100 to 150, each point of PRR is worth about 4hp, and from 150 to 200, 3hp per point.

Whilstthese are rough rules of thumb, they should prove useful in eyeball comparisons of possible gearsets and enhancements.