Brooding charges

I have been selling some started chicks this year, but not charging for the cost of brooding them. So people have been leaving them with me for up to the first 2 weeks and still paying the day-old price. This is definitely going to change next year! There is a lot of time and care that goes into getting those tiny hatchlings safely to the age of 2 weeks, not to mention the cost of feeding them! Now I am being asked to keep them even longer in order for the buyer to make arrangements to pick them up. So this batch of chicks will be 3-4 weeks old when picked up.
So my question...is there an accepted rate to charge for brooding? Is it calculated by the day or by the week?
Thanks for anyone's thoughts on this!

I charge $1/chick/week. You could break it down if you needed to for partial weeks. They dont eat a lot the first week but I still charge it because you are right, it is a lot of work to get them through the first 2 critical weeks.
I had lots of people doing that to me and when I started charging brooding then they were more than happy to pick up the chicks on time.
Other's do not mind paying the brooding fee and have me raise them for a couple weeks to ensure they are off to a good start, so is nice to offer both options to people.

I also charge $1.00 per week for brooding. Food is not free where I live.

Thanks Hidden River and Dan. No, food isn't free for me either!
I did a bit of investigating after I posted that and found that Miller Hatchery charges .10 per chick per day. So that is in the same ball park. Seems to be a good incentive for people to make the effort to pick up their chicks, or have the option to leave them a bit longer and help pay for the feed and heat etc.