Yeah, I believe part of the point of using Eventbrite is to try to make sure that people are fully paid up well before the game starts, which has been an issue before.
When I was involved with the finances for the first Crucible game, we were generally pretty happy to consider someone fully registered if they had contacted us about a payment plan, and made the first instalment. That showed they were serious about attending as a fully paid attendee, but perhaps just didn't have the spare cash to make one lump sum payment.
In terms of full/partial refunds, I too agree that it comes down to the cost structure. My personal feeling is that for a weekend game, probably anything less than two weeks out from the game should receive, at best, a partial refund, because by that point, a lot of money will have already been spent assuming that the opening in the registration list has been filled. Not just on venue and potentially food costs, but also in terms of props and costuming and the like. Pulling out late might mean that there isn't enough time for the GMs to be able to wrangle someone else to fill the spot, meaning that they'll be down money in a few regards.
Also, if it's a game where absolutely anything is personalised to the participants, it's quite possible that the GMs will have spent effort they could have directed elsewhere on creating elements of the game focused on that participant. No/partial refunds also help to ensure that the efforts and time of hardwork GMs aren't wasted on no-shows.
All that being said, I'm not against full refunds for exceptional circumstances.