It is not a good idea to spend this much money in a campaigns rather you would have made different campaigns of $5-$10 each and observed the clicks, then chose what is performing better and go from there.
my 2c:
stop using all these shitty middle man services. they're probably just ripping you off that's why you get no conversions..... for $500 to get nothing back is horrible and practically unheard of if the traffic is targeted and real!
contact webmasters directly and setup a media buy. only buy on trusted websites even if it costs more CPM the results will be better..
some huge trusted sites may have a $1000 minimum. you can usually find out minimum ad spend from the sites media kit or by talking with the webmaster. to get around this say you want to do a test buy before you start buying serious volume and that you do intend to meet the minimum.
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my first media buy was on a large gaming website.. I was promoting a world of warcraft guide off CB - using the ads provided on CB, no landing page. I spent $250 made back about $170 if I remember right. Make sure your traffic is TARGETED that's the most important thing then just work on improving your conversion rate with better ads, landing page etc.
I would take the experience as an expensive lesson. don't deal with middle men, instead deal directly with site owners, especially when starting out.