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If a cheap ad does well, then is an expensive ad from the same website/publication going to do much better? Maybe. Maybe not. There are so many factors and variables that may influence how well an ad works that trying to make predictions is very chancey.
It's a matter of reaching the right person at the right time with the right message. Just because one ad performed a certain way, you can't assume that a different ad, elsewhere, or at another time, will do better or worse.
Try it for long enough to give it time to work and come back and tell us what you learned.
Define "upgrade", are you talking about a sponsored link that's above the fold or something else?
$50/month isn't a lot if you have the right ad for the right service on the right site.
On my site it's a heck of a lot cheaper than Google AdWords when you do the math as most of my advertisers are paying less than $0.05/click.
However, even if my traffic doesn't convert for them, it's still damn cheap branding.
[edited by: incrediBILL at 11:08 pm (utc) on Feb. 21, 2007]