The item-creation guidelines suggest that a use-activated item of disintegrate (a 6th-level spell, minimum caster level 11th) would cost 6 × 11 × 2,000 gp = 132,000 gp. That would just be for an at-will disintegration stick, not attaching it to a weapon, and at a guess it’s on the low side (they are only guidelines, after all).
Another way to look at it, and this is actually what the books recommend (with the guidelines above only as a fall back), is to compare with existing weapon effects, e.g. all those +1d6 or +2d6 damage ones. A CL 11th disintegrate deals 22d6 damage on a failed Fortitude save, 5d6 on a successful one. So no matter what, that’s at least 5d6 damage, and it’s largely unresistable, so that’s actually better than +5-worth of bonus damage (since the various +1d6 and +2d6 weapon properties are all either conditional in some way or involve resistable energy damage). So even if we took only the damage from a failed save, that’s at least a +6 effect (minimum 72,000 gp, assuming you’re starting with a +1 weapon of some kind). Note that +6-equivalent effects don’t even exist under the usual rules. Considering how much more damage happens on a failed save, the number would have to be far higher.
It’s also much better than the various critical-hit effects; flaming burst’s improvement over flaming is in the 1d10 to 3d10 fire damage on a critical hit. That is, according to the game, 3d10 damage (assuming a ×4 weapon, and thus not an 18-20 weapon in most cases) is +1 equivalent. That’s an average of 16.5 damage. The 5d6 damage on a successful-save disintegrate averages 17.5, and again, is unresistable. So getting disintegrate-but-they-always-succeed-on-the-save on crit is worth about +2. The 22d6 of the failed-save version is, on average, 77 damage; easily a +5, probably a +6. And that’s comparing the ×4 version; if we compare the 18-20/×2 version, we’d need an enhancement bonus of three times that.
Or we could compare to vorpal, which is nat-20-only, but insta-kill for most targets (with some important exceptions). The 22d6 of a CL 11th disintegrate is not really insta-kill territory, but it does apply far more widely. A disintegrate-on-nat-20 weapon is probably a +5, but it’d be a kind of poor +5; someone might convince me it’s +4.
But investing in nat-20-only effects is a poor choice, seeing how rarely they happen. A 5% chance of 22d6 damage averages only 3.85 damage; +1d6 all the time averages 3.5. This comparison is a little unfair (the +1d6 still requires that you hit which may lower the expected damage a great deal), but even so if you hit half the time, disintegrate-on-nat-20 still only has the expected damage of a +2-equivalent. It just has to keep the higher cost because of the risk that a lucky hit invalidates an entire encounter.
Personally, considering that, I’d probably allow disintegrate-on-crit as a +5-equivalent weapon property. Yes, you should put it on an 18-20 weapon, and thus yes, the expected damage is quite a bit higher than five times flaming burst sans flaming, but it’s a very unreliable weapon and crit-fishing tends to be a weak strategy, so I’d feel comfortable treating it as such.