Not sure. My son went to a party at one of those pottery painting places and they came home with the pottery item they had painted and glittered. Perhaps check out anything local to you like that for party prices.
Also rather depends on what craft activities you are going to do with them. I run an after school art club which I charge £2 per child for but that covers materials only and is non profit, I don't get anything for myself. I tailor my activities to that price and could spend a lot more per child if I let myself go mad!
If you want some decent craft kit ideas where you don't have to gather everything togetehr yourself and which are quite cheap try yellowmoon.org.uk. I can recommend the dream catchers and the pencil toppers with dangly legs which are on special offer at the moment 1.50 for 6! Use acrylic paint pens if you're worried about mess from paint pots, a little more expensive and you have to ensure you have enough to go around, but so much easier than brushes, water, spills...
Yellowmoony also have some lovely fabric painting ideas, such as drawstring bags, shoulderbags and puppets. Or you cold get cheap t-shirts for them to paint a design onto.
I also suggest you ensure that you cover yourself by making sure parents aware that they need to provide an apron or old clothing (unless you have enough aprons to go around). Also make sure you have it written somewhere that you accept no liability for damage to clothing etc.
And depending on the numbers you might need an additional helper. So think about factoring in the cost of hiring someone else to come along too. Parents will probably want to sit back and leave you to do it all, so don't rely on them for help.
Find out how long the party is and how much of that will be spent doing the activity. Assuming they will need time for singing happy birthday and cake etc. And most importantly agree the numbers in advance. I would say give a fixed rate for a party of up to x number once you have workd out the costs. If less come then you still need to cover the cost of materials. And if someone extra turns up on the day, or someone brings a sibling then you can always say sorry it's for a max of 20 (or however many)
So lets say we assume the party will be 2 hours. You want to calculate your expenses as something like:
Cost of craft materials
Additional expenses (travel etc)
Paying for Additional Helper (or 2)
Amount you would like to get bearing in mind your time and effort
Total all those expenses, and round up a little for unexpected things. Then divide the cost per child and see if it seems reasonable in relation to what else is available around you. If you are horribly underpricing yourself, or are way over the top rethink your activities and adjust accordingly.
Best of luck
Gilly