Just make sure you focus on a few things well enough to track their costing. I'm still relatively new into breeding two colours of Orpington, two breeds of ducks and West of England geese. I think the only ones that cover their costs are the geese and I'm never going to make much from them because they only lay about 30 eggs a year and they're hellish difficult to hatch. I've found the hens much harder to shift that I had anticipated because everybody round here just wants some nice hybrid egg layers and my big, beautiful but expensive birds are not what they're after. Interestingly I've had more interest in the ducks but had a really bad year for hatching so I only had a couple of ducks to go and the rest were all drakes which will be eaten (sadly).
Also agree with the small batches being a problem - it's hard to anticipate the number of runs and houses you need if you've hatching twenty birds a month. But then equally if you're hatching hundreds you're on a whole different scale. Either way start up costs are huge.
If you're selling eggs at the gate or direct to consumer that's fine but if you want to sell to shops, restaurants etc. you need to be a registered supplier which involved egg grading and stamping. Remember the huge seasonal fluctuation in pure breed eggs layers too - I get one egg a day at the moment from about 18 'in lay' hens and two eggs a day from 12 'in lay' ducks.
If you want to sell meat you have to look very carefully into the hygiene regulations. You will need a hygiene certificate covering the preparation area (which means you will also need to comply with their requirements for e.g. handwashing vs. food prep areas, correct type of surfaces for clean down etc.) and for you, proper chiller facilities and chilled delivery facilities if you're not selling directly to the consumer. I think there are also new regulations regarding dispatch requirements. Personally I wouldn't go there because I can't believe it's commercially viable unless you have a local registered abattoir that would handle poultry (unusual and probably expensive). If you just want to eat lots of chicken, that's fine and will save you money on meat.
I think if you want to make money you have to choose exactly which of the various chicken related activities you want to do and just do that. Not a few eggs, a bit of meat and a few POL hens. That's what I do and it doesn't make money - food alone this year will have cost well into four figures (I think about £2k - I stopped recording when it became too depressing!) and I've sold about £600 worth of birds and probably £400 worth of eggs (again stopped recording in June when it was clear that even at the height of egg laying season I wasn't covering food costs - partly because I had so many growers). I will have saved on buying eggs and meat but even so, it's not going to close the gap - and that doesn't factor in bedding or vet costs either.
Just to throw another iron in the fire, if you can sort out the meat handling, Christmas turkeys would cover their costs. Mine cost £14 each in July, will have eaten about £60 worth of food between six of them and will be worth about £40 each on average (I'm guessing). The big scale local organic/free range producer that we have bought from in the past charges twice that.