I actually no longer think it was Spotify, default setting is only stream over wifi, so don't think it has been the cause.
you have 2 options under settings stream using wifi or 3g
remember these settings only applied if you have to sync music for offline not for streaming.
so while you listening music you basically streaming.. over 3g when wifi not present.
I have tested on my hero using spotify
best thing is you should sync music into offline mode using wifi and listen later from your offline playlist and that will give u music on the go without streaming...
instead streaming music on the go each track can suck about 2 to 3 mb depends on the quality of the sound and stuff.
never stream music over 3g .
even IPhone o2 customers dont stream that much on o2 even though o2 give them unlimited but again streaming over 3g not advisable 1st expensive, 2nd quality fair usage policy applied.
best thing again sync music using wifi to your offline playlist as much as u can create for a week or 2 or everyday using wifi.. then u can listen offline music anytime without using ur data connection u good to go.
also on hero when u browsing htc hero, images and data fonts get compress, the size of the webpages and images automatically compress and uncompress while transferring data between network and the host and same thing with the email.
if you download say 2mb email on nokia it will suck 2mb from ur data. however if u download email on hero or iphone or blackberry, data use slightly less say 2mb email will using 1mb as its compress the size of data while you downloading and uncompress once its download to your phone without effecting your using experience.