Let's run the math here. At DC 10, the spell is only likely to take effect when the opponent rolls a 1 on the will save. But that happens from time to time.
If we assume two hits per round, that's a 9.975% chance of causing an opponent to lose an action every round--if they only fail on a one.
If we crank up the number of hits to four (through haste and being higher level, two weapon fighting or whatever), that works out to an 18.55% chance of the opponent not being able to respond.
If everyone buys the weapon enhancement and we assume three more hits from the rest of the party that works out to a 30.16% chance of the opponent being dazed. EVERY round. If that's the case, you had best hope not to have too many single monster battles.
So, the net effect is that you nearly double the amount of dice you roll in every combat and bad guys get screwed by the law of averages. (Or good guys if the bad guys come up with that weapon enhancement too).
The long and the short of it is that I don't think it's a good idea at all. It works in Neverwinter Nights because 1. it's a computer game and computers can roll all the dice they want without slowing anything down and 2. The typical battle is multiple monsters per PC, and 3. the game designers fudged their way out of a lot of balance issues by simply making important bad guys immune to a lot of unbalancing effects behind the scenes.
If you do insist on doing so, however, I think the progress of the various surge weapon enhancements is a good way to evaluate it. In the DMG 2, they were all listed as plus gold enhancements. Sudden stunning, for instance cost an extra 2000gp. Someone must have noticed, however, that they were ridiculously overpowered and dramatically undercosted because by the time the MIC came out, stunning surge had become a +1 weapon enhancement (and it operated on a fort rather than reflex save and lasts only 1 round). In fact all of the on-hit abilities in the MIC that effect your enemies--even ones that require a swift action to activate are +enhancements rather than +gp. The only +gp adders in the MIC (other than weapon augment crystals) are aquatic, everbright, hideaway, prismatic burst, shadowstrike, sizing, slow burst, and vanishing. Prismatic Burst and Slow burst only function on crits and vanishing affects the wielder rather than the target.
So, having said all that, what would my take be:
Either
A: Daze (DC 13--based on daze monster) on critical hit. That should be somewhere around 3-4000 gp. (Call it 3,500gp if you want a concrete recommendation). It's not as good as slow burst but it's actually pretty close. Denying your opponent one action entirely is arguably better than slowing them for three.
(You'll note that my price is not explicitly based on a formula--this is because the formulas are really only guidelines for where to find a starting point for costing (as the designers themselves wrote in the DMG where they spelled them out). It is very possible to make dramatically undercosted items as well as ridiculously overpriced items using the formulas. The final step should always be evaluating it against other existing items and asking "would any character buy it at that price?" "would every character buy it at that price?"
B. Make it a swift action activated ability at a full +1 enhancement bonus.
Dazing Surge. Upon successfully hitting a monster, you may expend a swift action to daze the target for one round (Will DC 10 + 1/2 character level + Cha bonus negates). This ability is usable 1+cha bonus times per day.
(prereq: craft magic arms and armor, daze monster)
