I regularly hang out with people who have widely disparate incomes (meals after Critical Mass especially, combo of geeks and students) and the banker system works pretty well in that situation. Although we normally call it "the donation tin". We bulk order and accept donations, and the extra goes into the donation tin for next time or "other things" (Critical Mass does "other things" quite well). A while ago we did that after mass and made ~20% of the total in "profit".

I think it's partly because we eat at cheap places, and partly because there are enough geeks to cover the shortfall. It only takes ~10% of the people to pay 50% over the actual and you're square. This also covers the small change problem - few people are capable of both adding up what they ate and finding exactly $7.65 in their wallet, but for the banker that missing .15 or .65 adds up over 20 people. Plus here we don't have the hidden fee nonsense that you have to put up with, so people have a better idea of what they've "spent". We do have a degree of haggling ability though, and I gather that's also rare in the US. "haggling" and "asking for a discount" depending on whether you're asian or anglo. So if we get $220 worth of food but I only get $200 from the group I will often say "group order, how about a discount"... sometimes I get it. Other times, we just get a deal automatically, like ordering 30 pizzas for pickup at 7:30pm on a Friday (after Mass) we get the "Tuesday lunchtime special price".

I suspect you're also used to much more expensive meals than I am, which does change the dynamic. I'm quite willing to pay $200-odd in the expectation of getting it back but it's no great risk to me personally if I don't. But if that was $2000 I'd be much more reluctant to take the risk. I think partly because I associate that with much bigger gatherings where there are less direct personal links. I just can't imagine paying over $100 a head for a meal...