When I find several birds are 10 lbs live weight, I take in the whole batch. That usually gives me a nice range from 4 lbs to 8 lbs dressed weight.
The price for processing is $5 - $7 per bird depending on live weight. It's also a 180 mile round trip to that processor. I know of a place that will do them for $4 per bird, but it's about 400 miles round trip. So, I don't gain anything. This is because I sell my birds. You may be able to find uninspected processors to do your small batches for you.
Also, that $7 per bird barely covers the gas. If I start thinking about business licenses, business insurance, car insurance, liability, etc. then that $7 is way too low. So, I don't look too deeply into the number.
[edit] I do see people on here state they can get processing for $2 per bird in the midwest. I don't doubt it. Probably a lot more chickens are grown there than here. Heck, I pay $12-$13 for a bag of starter... $10 for a bag of cracked corn. Everything tends to be more expensive in the PNW.