If you are licensed to carry out work on said machine then you can, even if you own it - so yes it is potentially a lot cheaper!
PFA (now LAA) Permit aircraft maintenance cannot be signed for by a LAA inspector who owns said machine unless he is a CAA licenced engineer as well (and I'm not sure but it might have to be within the qualifications of his CAA licence - ie metal or wood etc). Build stages of a new machine would have to be signed off by an independent inspector.
It used to be the case that BMAA Inspectors couldn't sign for any microlight that they have monetary interest in.