The biggest charity ride (by far) around here is the Pan Mass Challenge. The entry fees cover the cost of putting on the event so 100% of the money raised in donations goes to the Dana Farber Cancer Institute for the work that they do. They also have a minimum fundraising commitment - and take your credit/debit card number when you sign up so that you will be responsible for any shortfalls if you haven't met that minimum by the day of the event. For the full 2-day event, the minimum is $5000. They can only accommodate so many riders so they don't want to waste slots on people who can't raise the money. There are shorter routes with a lower minimum as well. And a virtual event where you decide how far you want to ride and how much you want to raise and that has no minimum.
It can't be too big a turnoff because the slots fill up every year and they raise more than $50 million. And they are one of the best-run and highest rated charities in the country.