Welcome to httpforum,
There is no limit in the amount you can earn from PPC but it needs idea, content and most important patience !

I didn't have better experience than $10-$20 a day but currently I see people earning $300-$600 daily just from google Adsense.

Don't waste your money on carbon copied articles and prepared Adsense websites as the duplicated contents would never lead you to success.

Having a website with unique content or offering a good free service are the best ways to generate traffic who leads to PPC revenue.

For sure there are other temporary ways to earn without having anything interesting but those ways won't last long and will never continue to work on autopilot.( i.e. spending money on non related low cost ppc campaigns to get visitors and put high paying keyword ads on your site.)

I would be more than happy to discuss more about this interesting subject.
Cheers