I don't spend money in promoting my sites. Just the free stuff like social bookmarking and promoting in forums.
I normally take 1/4 - 1/2 of my site earnings and put it into different types of marketing like adwords, quality backlinks, etc. Sometimes less and sometimes more, it just depends on my funds at the time!
I spend $60.00 on one, about $25.00 on another. The one I spend $60.00 on gives me almost all my traffic, but the $25.00 one I recently started and the ROI hasn't been so great, if not 0.00%. Be careful where you spend your money would be my advice.
As PHPGator said, you should certainly be careful when spending money. Try to use money made to cover costs or you will find yourself in the hole real fast. Sometimes marketing helps but there is no guarantee that anything will come of it.
Two months ago just got started in the online thing. Will probably do at least $2,000 a month and it will keep on going up and up. Why are you asking, anyway?
After loosing several thousand only thing I learned is there is nothing better then do it yourself. Spending 0 and getting better results
About $60/day with adwords. Will slow it down around Dec 22 and then increase it back up to about $100/day in March. Back in the YPN monetization days it was about $800 to $1000/day on adwords!