In some (but not all) cases I think positive motivation would work better than negative. E.g. pay $30 when I create a goal, get it back when I succeed.
I’d be okay with getting this as Beeminder credit (that I could spend on premium plans or other goals, or maybe transfer to other users) rather than actual money (which would raise a lot of issues around fees, fraud, unusual use of refunds, etc.) It would be even better if I could earn it back gradually as I go longer and longer without derailing.
I think this could actually be better for Beeminder itself financially, since you would get the money up front and earn interest / be able to invest it immediately.