It has been discussed, and its something that i strongly agree on, only really for main championships though. Roboteers dont seem to be put off by a small entry fee when Ive asked, and those fees would be put towards something like prize money.
The other problem is roboteers signing up for major events with robots that arent built, or not likely to be comming - people get turned away and the others sometimes wont turn up. An entry fee would secure you a place and make you think twice if your robot is going to be ready.