I'm not sure if it's allowed by site rules, but I am sure it's a terrible idea.
First of all, if there's anything the slightest bit subjective, such as in a popularity-contest, people will get upset if they don't get the money but think they should.
Second, if anything is underspecified, such as characters vs. bytes or date of language creation or whatever, people will again get upset if they think they should win.
Third, many really excellent results involve collaboration/using parts of other solutions, so divying up the money could get difficult.
I would say that if you want to give a monetary reward, host it off-site, use sealed submissions and give really clear rules.