Depends on where you live, but I've found the price range for classes is $25-200/mo (yes, that is a huge range) for most non-mcdojo schools. I've never lived in a huge city though - LA, NYC, etc nor have I taken classes from martial arts 'celebs' (top ranking people in the space).
When I lived in small-town Illinois the price range was $20-100/mo generally. $100/mo was on the McDojo end of things. Now that I'm in Austin I usually find $45+/mo is common. $200/mo is the usual cap out here if you're not planning on training every day of the week (usually for 3-5 times a week is the range).
Remember, these folks are looking to make money. This is a business.
But, how much is it worth to you? Figure out how good your instructors and classmates will be and that will determine your price. Also depends on the amount of time you can train - 3 days a week? 5? An hour a day? Three hours?
Find what the local market rate is (shop around at similar schools) and then determine from there based on quality of the school (it's people) and the quantity of time you want to devote to going.
Variations on price modelSome people charge based on a flat monthly fee. They offer x number of classes a week and you go to what you can. Some do it based on a base rate and then how many classes you attend (once a week, five times a week, etc).
Some will add on other things. Testing fees, equipment fees, uniform purchase (you should be able to buy this for yourself elsewhere - if they won't let you it's 99% likely that it's a bad school unless they're discounting your tuition with the purchase.), etc. Sometimes there will be an association fee - don't be too put off by this, it usually goes to a larger national organization that helps make things better for your art's community (making sure certifications are given to those who deserve them, etc.), but do your research still.
Some will have you sign a contract to ensure you stay and pay for a certain amount of time. I really recommend not going to any school that requires a contract with them over two months, especially if you're new to the martial arts world.
Also see: What martial art should I study? and Choosing a martial arts school.