First, depends on your income and if you are making one. Some say 10-20% percent as a rule but 10-20% of what. 10-20% of nothing is nothing. 10-20% of 50000$ - 5-10000$. You can spend a lot of money on advertising but if it doesn't get the results you are seeking, it can be expensive.
Advertising can come in many free forms as well. There are many things you can do that wont cost you money but your time. For example, doing videos and posting on youtube is free. Blogging about your local towns of interest, holding first time buyer classes and making flyers, walking neighborhoods with a flyer that has your business card on the flyer and anything you can attach yourself to that will get people to notice you. These are just a few ideas. I have tried just about all of them. The key is really do you and find a way to get in front of people who want to do business with you and that can be free!