Cost and location.
Many times these rides involve getting to a location the night before and paying for a room. After that, and meals, it can really start to add up (Yes, I'm cheap). But, whenever possible I try to incorporate a weekend vacation into the ride and spend the next day visiting and exploring the area we're at.
I understand there are many costs in putting a ride together. Paying the police, insurance, permits etc. But I still cringe when I'm asked to pay $75+ to participate.
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canklecat , I agree with you assessment of "charity" rides. People would be better off just donating their money. I've always considered them nothing but a feel good fix for the enthusiastic participant fighting for that cause.