I think its a bog standard 15% of your take home pay? I had to pay CSA for 11 months in my IVA and this was £170 PCM. My IVA was reduced for this time by that amount - but capped on the end...So it took me longer to pay the voluntary payments into the IVA. My son was 18 when I left home and I had no idea I still had to pay. - the CSA was a bodyblow at the time.