If you were asked to help someone restore an old quilt how would you go about determining how much to charge? Obviously the more work involved the higher the price, but what method would you use to figure out an estimate?

All you could do would be to set a per hour rate ... and make sure they understand the number of hours it may take, and that you have no idea for a firm quote on it.

Here is a link that might be helpful for you in determining the time going into in case you haven’t already repaired an older quilt. I’ve never repaired one so can't tell you from experience the time involved but found this link that might be helpful for you. http://www.quilthistory.com/repair.htm I skimmed over this quickly without reading all of it and it does mention that a lot of time goes into repairing a quilt. I guess that really depends on the damage and how extensive it is. I’ve also heard of an ‘honest’ patch, that is when no effort is made to match the damaged fabric.

you need to really look it over and figure out what it is going to take- write up a "plan of action"- including if you will be purchasing new fabric, batting, backing or anything else- such as fusable webbing...
do you have to re-quilt it? or pay someone to quilt it? machine or hand?
estimate the amount of time each step will take- then decide what you think will be fair to you for all your work and her--
i have restored baby quilts for $75, i have restored larger quilts for alot more- and a couple for alot less- you have to first have a plan in order to have any idea what the cost will be- i always tell people that is the only way to determine if it's fixable and what it will cost-and i try to guestimate a bit on the high side so they are happy when i can give them a little discount (like finding new batting/backing on sale-using a coupon)

As a former textile restorer I have to agree with ckcowl. I would add - give a written estimate and make sure that they are aware that if they don't pay they don't get their quilt back. Sounds very hard, I know. Luckily I was never "stiffed" by a customer but I have friends who were out £100s. And keep in mind that if you have underestimated, that you were adding to your skills and doing something that you love.

I'd size them up and decide how much they could probably pay and then increase that by a couple hundred.