S'mon said:
If you _were_ wanting to allow this, given that continuous-short-duration merits x4 cost, continuous-instantaneous merits at least another x4 for total x16, around 260,000gp would be reasonable.
That'd be reasonable only if you have some strange definition of the word, so that reasonable is equivalent to worthless. At the levels when a character can afford a 260k item, the ability to do 17.5 per round (assuming the enemies don't have SR - most high CR monsters will ignore CL 9) is fairly laughable in serious fights. And the characters will a healthy reserve of lower level, but still effective spells even after they run out of big guns. My high level wizard picked up a wand of Maximized Magic Missiles from some foe. I ended up using it a handful of times over the campaign - I believe the wand came out when we were fighting Spectres (so I needed Force damage, which most of my other killing spells didn't do) while we were on the way to the main boss, so I was saving up my spells (turned out to be a waste, since my character got Power Word Stunned along with other bad stuff so I didn't get a chance to use very many).
To some extent, the answer depends on your campaign. If you have tons of fairly small battles or very long fights, then having an unlimited spell attack is good. When the group does have lots of small battles each day, do they come in big groups, making area attacks more attractive? How often does the PC run out of spells now? Do many monsters have SR? How many enemies will have access to Shield potions or spells for extra AC? Do the characters have access to strategic movement or safe havens so that they can rest when needed?
Similar items are also relevant. For the price of limitless magic missiles period, the PC can get like two and half wands of the same spell. Since a free resource will get used more than a plentiful, but still limited one (rats in the sewer?! Yuck! Time for some magic missiles), we can probably ignore the extra half a wand. So that's 100 charges of magic missile, which is quite a bit. If we assume the BS standard 13 encounters (we'll assume combats), and that each fight lasts 4 rounds, he can use the magic missile wands exclusively for two entire levels and come out the same as the guy with the gauntlets shooting unlimited missiles. If the character uses real spells or attacks (I suppose that it doesn't have to be a caster using it) half the time, it takes 4 levels before command work pulls ahead.
Also, the price guidelines for a spell an unlimited # of times per day produce the same result as the same spell 5/day. So, in my game, that unlimited CLW item becomes a 5/day item. Of course, since magic missile is going to require a combat action, unlike a cure, there's a more realistic constraint on how many times it can effectively be used. Especially since using means not using Slow or Evard's Black Tentacles, or some other nasty spell. But if you forsee the PC firing like 20 shots per day from the item, then by all means limit it to 5 shots per day.