First let me answer the question this way. You should spend the amount you can afford to comfortably keep spending money for about 8 months before you get any money back.
Why is that you may ask?
Well think of it this way, the average agent will close a deal within his/her first 8 months of being on online leads. That's average, this means some agents get a closing within their first month, some in their 3rd month and some it could take them 2 years before they close a deal.
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Is it enough, should I spend more?
So, first, let's say you are spending $5/day, that's about $150/mo. If you cost per lead on average is about $10/lead you should get getting about 15 leads per month from that ad spend.
Depending on how long you've been doing this, you may have come to an understanding that you close 1 deal out of every X # of leads. For an average agent that X is 100 leads. So if this was true here as well you would need to spend $1500 to get one deal.
So when you ask your self, is $150/mo enough, think that it will take you on average 10 months to close 1 deal. So again, if that's all you can afford then yes, that's all you spend, but if you can afford more, do yourself a favor and cut down on the amount of time it will take you to close a deal. If you spend $300/mo it will take you 5 months to close a deal, if you spend $600/mo you should be closing a deal every two and a half months.
If I spend more per day, does Google waste my money?
No, spending more per day will allow your ads to stay on Google for a longer period and will get served to more searchers. Google bills you on a per click basis, well actually it's per impression basis but they show you that they click on a per click basis. If you spend more per day you should still continue to get the same cost per lead on the keywords you are targeting.
It's only when you get into spending very high amounts per month that you now have to start getting creative and maybe targeting more competivie keywords to get traffic and thus having to have a higher cost per lead.