Our local 'disco man' charges about £50 for the whole evening - we do 2-3 discos at 1 hour each, with gaps of 15 mins in between to get one lot of children out and the next lot in.
We charge £1 for tickets, this includes entry to the 'lucky draw' - we offer 2 small prizes per disco. We wouldn't do pay on the door because of security reasons - we have to know exactly which children are coming to the disco, who is collecting them at the end, and have adults' contact numbers.
We send letters out about 10 days before the disco. There is a cut-off slip at the bottom which must be filled in with child's details and details of who is collecting them. They bring in slip with £1, teachers collect them in and PTA members go round the classes every morning and collect them in. We then send home a ticket which has child's name and class no. on the back. When the child comes to the disco we collect in the ticket. We have a typed list of who should be coming (I do this using a spreadsheet, doesn't take too long) and tick the names off using the tickets. Then at the end no-one is allowed to leave without an adult, and again we tick the names off. We have two doors to our hall, so adults are let in a few at a time, collect child and leave by the other door, so it's a one-way system and doesn't take too long. This system sounds complex but it's really simple and is the only way we're allowed to have discos - before we took over, there were some nightmare discos where no-one turned up to collect children and where parents even refused when a teacher or the head rang them - this was on dark winter evenings and the parents were just saying 'let me come home themselves'. So we now have a really slick system going and it works a dream.
Because we're collecting money in advance, we always have enough petty cash to pay for the disco and to buy the tuck shop goodies. Make sure you have a float for on the night, and price things nice and simply, you don't want to be finding lots of 5p coins or smaller for change.
You must charge for refreshments - that's where the real money comes from! Although our school is fairly big on healthy tuckshops etc, this is the one time we're allowed to sell sweets etc. We don't go for really cheap and nasty ones, but go to somewhere like Aldi for multipacks of crisps and a few sweets. Drinks - we only do water bottles and cartons of pure fruit juice, these really sell well. If you're having two discos, make sure you keep back enough sweets and drinks for the second one. We also tell parents roughly how much things will cost, so they know not to send too much money, we get parents sending children with £10 notes! One year one boy had loads of money and kept coming back to the tuck shop, bought loads of crisps and drinks, then at the end came back tearfully and said he was supposed to take some money home!
Make sure you have enough helpers, and if possible get staff on board. Children love having teachers there, and it also adds a bit of authority 'just in case'. We are very lucky, we usually get at least 2-3 teachers or TAs staying for each part of the disco. We find we need 2-3 people to serve at the tuckshop, and a few more to tick off children's names, guard the doors and just generally oversee things. Setting up is really quick (we don't decorate the hall at all), it's just a case of sorting out the tuck shop and opening the hall for the disco man. Clearing up at the end takes a bit longer, so make sure some people are willing to hang around for that bit. We stop the discos 5 minutes before the end time, do the lucky draw, then get all the children to find their coats/bags etc and sit in class lines. Then when the parents come to collect them it's relatively organised and they can find their children quickly.