It varies depending on the standard of work that the MUA can provide. Some will charge hourly, some charge my each set. Have a look around, find a local(ish) MUA and if you like their work then ask them for their rates. IF you find a MUA that is looking to update their portfolio (rare but they are out there) then maybe discuss covering their makeup cost or at least putting some money towards. But generally a MUA will make a massive difference to a shoot, as they can do things that a model nay not be able to, and spot little details that need taking into account that a photographer may not notice while shooting.
Cost very much depends on whether you want a MUA to pop in and give you a make over, or hang around all day, constantly changing your look and touching you up every now and then.
If you just want one for an hour, and there's a good one near the studio/location, then she/he might do your make up for £30-£50. But a serious Pro might not get out of bed for less than a hundred.
Alternatively, you could try to find make up course in a college near you. Their students always need people to practice on, and most are desperate for pictures for their portfolios/course work. Although be warned... not all of them are brilliant. I used a new make up artist recently, when my regular one couldn't make it. She made my model look like Coco the Clown.
This was covered recently in a post here : http://www.purestorm.com/forum/readThread.aspx?id=156027
although very valid points were mentioned here in this one.
The best thing to do is contact and MUA in your area who's work you like and ask what they would charge and what you will get for your money.
You COULD as has been mentioned ask at colleges for students but as has also been said you don't always get great make up, just because someone is learning the art of make up it doesn't mean they will do a fantastic job for nothing, I would say ask for some images of past work and it will give you some idea. Most MUA's should have a book of their work.
It was nice to hear from the photographer here, how he values an MUA, yes we do do more than just apply make up, and style hair, we can pick up on things on set and make adjustments during the shoot. It definitely makes a difference to the qulaity of the end image.
Hope this helps