In the UK the government has said that PAF data should be made free[1]. I'm painfully aware of the almost extortionate nature that Royal Mail operate.
Having worked with Royal Mail PAF API, I known a 'friend' (wink wink) that created a class wrapper around the APIs. This 'friend' of mine built a custom Importer that automatically ripped all the PAF data into a MS SQL database. Post the data import, he no longer needs to renew he's licences because he is no longer using PAF data.
This may be something you could do also, buy the data one time an import it.
As for data changes, you can buy perhaps every few years e.g. 2-3 years and do a update of your existing data.
[1] Damn It! guess I was wrong, http://www.guardian.co.uk/technology/2010/jan/22/postcode-petition-fails-blocked-number-ten