Quite, small, light, not a rubber dome and cheap is going to be a very hard combination to find. Personally I haul around a HHKB Pro but that fails the cheap part
But it is quite, small and light but in a sense is a rubber dome but not a rubber dome at the same time
I have not tried one of the Cherry 4100 boards but they are supposed to be decent. They have the cherry MY switches in them and they can be had for reasonable cost. For instance here is
one with a BIN price of $39 with free shipping check out the picture on that one to see what it looks like. If you look around you can find them cheaper but you have to pay shipping.
Like someone else stated the HHKB Light may work as well but again I'm not to sure how it feels and it is a rubber dome. But a auction just showed up with one with a starting bid of $19 at that price it may be worth trying.
It should also be noted that all of these keyboards are PS/2 not usb but you can use a converter on them.
I'm interested in what you end up getting in the end because there are so few options out there for small and quite no matter the price.