Sketching a few ideas on the back of an envelope, I can't see that a CVR is any big deal from an Engineering viewpoint. I think with off-the shelf parts I could get one fitted into a light helicopter for less than £500 and 1kg. Add in regulatory costs, some crashproofing and a profit margin and you are probably looking at £2-3k and perhaps 2kg. I don't think that there are many small helicopters which couldn't stand that, although in the time honoured fashion it would be best for such a thing not to be retrospective. I'm thinking of something running, say, a 2 hour continuous loop recording from the intercom.
An FDR on the other hand is a totally different kettle of fish, because of the need for data to feed it. The easiest way to do that would probably be use of an uncertified LCD EFIS unit in the copilot/pax seat, which records basic flight data, with a certain amount of crashproofing around it; I'm thinking of something like
this which I've flown with a bit and whilst I dislike the ergonomics a great deal, is small, light, under £2k, and does maintain a limited 25 sortie flight and engine record.
Both would be crude and no substitute for a proper CVR/FDR system, but such a full system would probably cost more than a reasonable second hand R22.
Mind you, you have to ask yourself whether there is really enough incidence of small helicopter accidents, where the various BOI really struggled that badly to work out what had happened, to justify it. I read all the UK reports, and I don't think that's the case - it would be expenditure and complexity for the sake of it, and I have trouble with that as a concept.
G