I have a couple of 100 watt lasers, but they are in no way lower cost. Mine are US made Universal Laser Products machines. New, they are in the $40,000 range. What I funny, is that I have been in the model railroading hobby for nearly 50 years and have owned lasers for the past 7. I have rarely ever used my lasers for hobby related purposes. 

To answer your question, the 5-10 watt machines are more for engraving rather than cutting. There are cheap lasers from China that are sold on ebay. From my understanding, they take quite a bit of fiddling around with them to make them preform to the level that a modeler would be happy with. The other issue with lasers is that you have to have a way to exhaust the fumes from what ever you are cutting or engraving. If you are going cheap, I would suggest budgeting around $4-5000.