I agree. $20/mo is definitely going to be too expensive for a ton of potential customers. I guess it's a luxury that 37Signals can not only afford, but will maximize their profit and I would assume they have some good historical data to back up the decision.
I do think that there is a magic $5-9/mo range that individuals will pay without thinking too much. Above that amount, they start equating a subscription with a trip to the movies or a (cheap) night out and you've lost them.