After 2 seasons of demoing I found that the price was in the range of $25 - $60 depending on where you were demoing.
The most efficient way to demo is to have a short list of the skis you wish to try and then locate the shops that have them.
Another good thing to do is find shops that are either in a resort town or on the mountain that do a multi-demo. This means you pay them anywhere from $35-60 and get to demo as many pairs of skis as you can in a day. Places like Northstar in Tahoe and Beaver Creek in Colo have on mountain shops that offer this service.
The biggest downside to demoing skis away from the mountain is that you are stuck with 1 pair. So if they aren't to your liking you're screwed. A lot of times if you demo up at the mountain and get out on a pair of skis and don't like 'em you can take 'em back and trade 'em in for something else.
Another thing to do is to contact the ski companies to see when their demo days are. Most companies still host demo days, they just don't seem to advertise them as much on their websites any more.
I have been at resorts where Rossi had demos and all you had to do was fill out a release, leave them your driver's license and you were good to go.
Additionally, a lot of the small companies (PM Gear, Moment, Prior, Icelantic, Fatypus, etc) will host numerous demo days in their given regions. Check with them via their websites to see if/when they are demoing.