It wouldn't be worth doing.
Average catch system is £100-400 per month, £15 a month for nominet membership/dac, then the reg fee on top.
If you pay per catch, looking at £5-10 per catch depending who your with, +£15 nom/dac as above + domain reg fee.
The costs for someone who wrote their own system depends on the server costs which can be 100s, the IP lease, nom/dac costs+reg fee.
When a private catcher offers public slots, its usually to try and offset their own costs, a few quid wouldn't be worth the hassle.
Edit: Domainview, where Rob offers chasing, drop catching and lists which all offset his costs is quite cheap but don't think £20 cheap.