Personally I think it is a sign of the economy for the past several years. More and more people seem to have gotten in to the landscaping and tree trimming business. It seems that when we are home there are a lot more people that stop by to want to cut our grass or trim our trees. I just tell them that I handle those things and shut the door.
I'm not sure if it is a scam or not but if the guy was a legitimate business, he would have liability insurance and worker's comp payments, in addition to his other business expenses, to figure in his quote. If you got a really cheap quote, chances are the guy is a fly by night hack without any insurance which can come back on you if anyone gets hurt.