I have worked with several rides in the Houston area, both "fundraisers" and the "pay your registration and ride". I don't really see a whole lot of difference.
The three fundraising rides all have a minimum amount required to participate. Depending on the ride (they are two day rides), this is anywhere from $150 to $400. Just look at this as the registration fee, pay it, and don't bother with fundraising. Now, of course, if you want to give the ride more than the minimum, they will be happy to take your donation. Guess what, the day rides that have the $40-$50 registration fee will also be happy to take additional donations.
Jeff