The batteries last ages, and you'll know when you want to replace it. Its a good idea to take spares gigging and whatnot as they only take seconds to replace. The only problem i've had is that some 9 volt batteries are slightly too small for the socket, and so the signal cut out quite regularly...
Thanks a lot for the info, I already have the bass. I bought it about 3 years ago for £350 (probably around $700), and haven't regretted it at all. I just wondered that now it was 25 years old if it would have increased in value.
Thanks!