Depending on how many other belts and if there was no additional damage, as well as who your mechanic is and where you're at, I'd say anywhere from $250-$450 is about right. Don't be afraid to call around to a couple different mechanics and get estimates over the phone. If the guy won't give you a ballpark figure over the phone, don't use him.
Most reputable mechanics will be able to give you a parts AND labor estimate, going by what thier little book says the repairs should take time wise. (labor is charged per hour) So, basically you need to find who charges the least per hour for labor and the rest should be all about the same.
Replacing a timing belt is a labor intensive job, particularly on smaller cars, as they practically have to take apart most of the engine to get at it.
I have had them fail and cause no damage other than the car won't move, and also had them fail and cause serious engine damage (it was over a grand!). Most manufacturers recommend they be replaced between 60 - 75,000 miles, most of the time when they fail there is no additional engine damage, the car just won't move.
I will often have the timing belt replaced when doing other work like replacing a water pump or alternator - since those have to be removed anyway to get to the timing belt. Capitalize on the labor expense.
A colleague of mine hates his Kia since parts are not cheap - for some reason they seem to run 10-15% more than similar parts for most other imports.
It wouldn't surprise me at all if the timing belt bill ran over $600, also depending on the other stuff being done.
But, get your estimates from folks that do lots of engine work (stay away from the guys who do mostly oil change/exhaust/brake/tires etc. for the majority of their business) Engine guys will be able to finish the job right, and on time, and often can cut you a better rate on the labor since they know they can cut some of the fat out of the time in the book since they know what they're doing.
I'm still unclear on something...is it typically cheaper to replace it before it goes, or not? I was thinking of having this done on my 94 Accord; what should I expect to pay? It's strictly preventative (i.e. the belt is working fine at the moment).
It's almost always cheaper to have it done before it "needs" to be, particularly since if you don't do it before you "have" to, when it fails it could cause other damage. Also, there is value in having it done on your own schedule as opposed to needing to get it done - the inconvenience of not having your wheels for a day or two can be quite the hassle.
My overriding point was that while it is recommended around 70-75K, because of the expense, it is well worth having it done well before 70K if you are having other engine work done like replacing an alternator or water pump.
As far as your 94 Accord, depending on where you live and what local labor rates run, and what else you might need done, it'll probably be close to 5 bills.
If memory serves, there are some vehicles (I recall a Nissan I owned once) that do not use a timing belt but instead use a timing chain - these almost never break under 100K, and if they do they won't cause other engine damage. There's probably a lesson that can be learned in that for some vehicle owners about how to plan for the big expense.