Simply put, adding a monetary reward won't encourage participation. If anything, narrowing it down from multiple competitions to one "challenge" actually lowers overall participation, especially considering that neither Grey, Vegetto nor I can draw. The competitions didn't have good individual participation, but it had a better overall participation rate than these challenges as they had a wider spread of skills that tailored to individuals specific skills, like my or Vegetto's graphic design skills. In conclusion, The challenges are actually a step BACK from what was intended, as it only tailors to one skill at any given time, which greatly reduces the number of capable participants.
So the solution in my mind is to either revert back to the competitions or develop a single challenge that can utilize more than one skill. Maybe a team competition? I'm thinking of something where it has, like, this central theme (Mario, for example), and we have teams of up to, say, 3, collaborate on making one extremely badass piece using their individual strengths to augment each-other. (i.e you draw Mario and Browser fighting and shit and I take that drawing and translate it into a graphic design) or so on and so forth. Though as a rule, it would require the use of two different "skills" (which would have to be elaborated on) and have a minimum team requirement of two people. You know, so no single person just goes and hogs all the prize moneys for himself :L