Where do you live? In Seattle it's around 8x the cost.
A basic ticket is 13.50 (if you buy 10 in advance which can then be activated at any time, as is most common) to 23.00 DKK (if you buy a single one in a machine or via a text message); it lets you travel for an hour and change as much as you wish. If you travel every day between home and work you'll usually buy a flat-rate subscription.
(The prices get higher if you travel through multiple fare zones -- there are almost 100 different zones that cover Zealand and its 2 million inhabitants; it can be a somewhat confusing system sometimes resulting in travel from A to B not costing the same as travel from B to A; if you didn't buy enough fare zones you're also liable for that 750 DKK penalty above)
Bus travel require you to show a ticket at the entrance, so there's rarely secondary control, but the light rail system you have to buy or stamp a ticket yourself I guess has perhaps a 15% chance of being checked. I'd estimate at least 98% people have valid tickets on the lines I use.
There are a couple of caveats. When I don't have a ticket, e.g. because I forgot my wallet, I'm on a constant lookout and the train ride becomes really stressful. If you get caught several times they might press charges against you, so it is not feasible to go without ticket on a regular basis anyway. And last but not least, I'm using a service, so I pay for it.
I used to ride without a ticket a lot when I was in school. You are right about the stress. (On the other hand, riding with a ticket afterwards is much more boring.)
http://www.npr.org/blogs/money/2010/05/dont_pay_your_fare_on...