It´s hard to say, ppc price depends on your keywords. But you could do some math, lets say you have a click rate of 1%, which would mean you recieve between 250-300 clicks on adsense ads. This rate can be way lower for some sites and higher on others. Depends on type of site (forum is always lower I think) and ad placement of course.
The price depends now on your keywords, take a look at how much you recieve per keywords at the moment. If you recieve lower clicks per day you should probably work on ad placement and appearance.