I used to budget around $400 per month, which covers my classified and Yellow Pages adverts. Yellow Pages charged me about $80 per month for my ad. This was in the early part of my career.
Your situation will be different to mine as you may have more or less competition than me. But the above figures will give you an idea of what I spent in my first year. Although it may seem a lot, just consider the business that you are bringing in. In the early days, after a few months experience, my typical month earned me around $4,000, most of which was profit.
Over time I started to cut back on my advertising because I was getting more and more repeat business from existing clients. I ended up with more in my pocket and less expenses but it took me a little time to get there.
While this was my approach, with hindsight I recommend you do the following: slowly increase your advertising spend! Why? Because you can then start cutting out the less profitable clients (who you may be charging at a lower rate). This way, every hour you spend with a client should be at an ever increasing rate. Time is limited so maximise your return when offering your consultancy. In addition, as you grow in experience your value to the client increases and you should be paid accordingly.