You can set a daily budget amount. Five bucks a month would only be 16 cents a day. You could bid that low for keywords that don't have much competition from other advertisers. But that is a very tight budget for getting your ad placed anywhere in the top 10 positions.
I have used adwords and it is effective but not as effective as I hoped. I only had a few click throughs per day even though my ads were appearing for hundreds of keywords in quite a high position on average.