msr.
I cant think there would be too many 'active' 5.1's for under 500 notes. When you say 'active' do you mean powered speakers & subwoofer?. Powered speakers are usually expensive items due to their on-board amplification, although they have the benefit of doing away with big-bulky power amplifiers. Active subs are more realistic on the wallet.
Powered ranges usually come from the 'pro' studio environment
(haven't really seen many 'active' Home spec units).
Powered speaker makers include:
M&K, Dynaudio, Mackie, Circle, Alesis, Spirit, Genelec & Adam.
These would set you back a nice little sum, possibly about £500 for a stereo pair !!!! upto about £3000+ for a stereo pair.
Or am barking up the wrong tree here?
Adzman