I just went through the trademark process with my business. If you use the abbreviated form the filing fee is $275. http://www.uspto.gov/web/offices/ac/qs/ope/fee2009january01_...

Costs beyond that are lawyer's time for doing a trademark search and filing the paperwork. I used a lawyer (and ended up spending about $1,200 including filing fees), but you can also file it yourself if money is tight.

I think it's definitely worthwhile. The odds of having to make use of your trademark are low, but having the trademark is extremely valuable if someone comes out with a similar product or service with a similar name.