I would suggest that a reduction in annual maintenance from £4000 to £500 will be due to rather more than a change from a CofA type to a PFA type!!!!
One problem is that if one is going to buy a 30 year old plane for say £30k (as most people do) then the maintenance cost WILL be high. That's the whole reason the thing cost "only" £30k; it's because the previous owner(s) dumped a few k into the thing each and every year that the thing is still hanging together. Such is the "maths" of this business. A Vauxhall Viva would last 100 years if you spent that much money on it.
Whereas if you buy a similar plane but newer, for say £60k, then ON AVERAGE the maintenance cost will be lower.
Ultimately, the only way to make a dent in flying costs is to buy something that you can maintain yourself. On a CofA type that means becoming a qualified maintenance engineer (LAME for G-reg or A&P for N-reg). A Permit type is the other way, but they are limited to daylight VMC and in other ways which may or may not matter.