Full-size Roller Coasters can cost anywhere from one hundred thousand to over 10 million dollars, depending on the size, manufacturer, etc.
If you're really lucky, you might find a really cheap roller coaster that's about to be torn down for a couple thousand - for example, the Little Dipper from Kiddieland in Melrose, IL was sold for 5,000 bucks.
Of course, that doesn't include construction fees, which can be somewhat more expensive. However, Schwarzkopf claimed that two people could set up a 'Wildcat' portable coaster in just under a day.
But if you want the real stuff, you're looking at millions. Intamin or Bolliger and Mabillard good for steel coasters. Great Coasters International or the Gravity Group are good for woodies. Of course, that'll be a multi-million dollar investment, plus a whole lot of paperwork.
Not to mention maintenance and electrical costs as well.
Infact the local theme park such as Alton Towers or Drayton Manor usually spend about 200 million dollars on their roller coasters.