If you want a really lightweight bass guitar, then the Aria Sinsonido 690 is your friend. Looks fab, weighs nowt, sounds good. Fretless model available. They come up here every now and again, about £150 for a decent one.
Or you could get an Ashbory, or an Ibanez GSRM-20... the GSRM-20 is very short-scale (28.6"), decently made, looks like a four-string guitar, fits a guitar gigbag, weight in proportion. It's a pretty good deal for the price, too. (But if you get a new one, change the strings ASAP... the factory ones are not good.)