Most people I know that need to keep their poultry on a business footing only keep them a year in lay at most and resell while they've a decent laying future for the next home to be worth paying for. They get £5-6 a bird at that stage and if they are genuine free range of particular breed then they make a real effort to say so and get the backyard eglu kind of lifestyle buyer that will have 3 more as pets for the kids and a few eggs than a business.
Big commercial units cull at a year too, so that's obviously the peak of lay after which it will always ease off. Those are the bald birds you get through BHWT and other rehoming organisations, still 2-3 years of declining lay but worth a few quid rather than nothing.
Laying birds aren't raised to be meat birds, they have other breeds for that, so you're not going to get a good layer to 3 years on full lay and then a decent meal aswell, just not how farming works now - there may be some more trad 'dual purpose' breeds about but if you've gone for high production layers it's not likely you'll have those.
So really if you're going to be business minded, think about a second batch coming into lay before your older birds go into moult and advertise/sell what you have while they still have value to the next in line, rather than keep til they're laid out and then expect sales or meals out of them. They'll be scrawny picking by then on either job at a guess. Folk I buy day olds from have two sheds, one August and one February, changed annually and birds sell at £6 (central belt Scotland) at that stage with no problem.
I think £6/bird is steep- you can buy POL brown hybrids for £6 each round here from a commercial rearer. I paid £6 for POL white Leghorns too. One of our friends is a chicken farmer, they sell theirs for £2/bird (used to be £1, went up last year) which is more realistic. I have a few each time, they go on for another couple of years.
Not sure where you are but this is just what my supplier of day olds told me she gets for her birds at that point, over near Loch Lomond. And I've seen similar prices, around £5 from a local (Falkland) organic farm shop selling off layers around the same age. The first has a mix of marans, sussex, bluebells, rhode rocks, blacktails, speckledy and welsummers, the latter is mostly some kind of commercial brown hybrid as far as I can see, maybe they get more because of the organic label and the reputation of the shop. No idea, but that's what the woman said she gets and I have no reason to challenge her.
For comparison the many local spent hen welfare groups ask for 'donations' of £4 a bird at the same age from large commercial flocks with hardly any feathers, even/especially the commercial free range birds. I had 4 via doganjo a few years ago and wouldn't refuse more at some point but they really had no laying capacity like the ones that have been genuinely free range for that year. No recovery time, no thin shells or shell-free eggs..
Up to the OP what she charges of course, I've no idea where anyone else is or what the norm is there, but it would be odd if the birds cost £2 at POL and £4 a year later in that condition..
My own birds die here of old age, always have, usually with their head in the feeder or else still clinging to the perch in the coop but tipped over. I'm