The only advise i can give is Clean and replenish as often as possible throughout the event, stay away from the tanks if you can. I would generally suggest toilets cleaned 3 times daily and showers twice daily, staff working a straight 10 hour shift can also empty waste bins. Check with trolley access to the toilets, if the loos are raised the cleaners will have to leave the trolleys outside, and your stock will go missing, sometimes the revellers may try use the toilet rolls as streamers and the toilet cleaner to light the BBQ's. Think about PPE for the staff depending on the festival, needles and sharps, also surfaces that may be used for sniffing Drugs.
Are you providing the consumables, Feminine hygiene bins and vending machines, Are you looking after 100 cubicles or 100 port cabins, how many showers, if you want some help on pricing please let me know. Also are you providing the any other services to the festival, may be worth thinking about if you are on site anyway, car parking, security, first aide ?
PS a good earner is the post event clean up, litter picking and deep cleans on portaloos
Do not be scared to ask for a good price, you may have to consider how much time you will need to put into the planning and delivery, make sure it will not impact on your other customers, short term gains can lead to long term pains
Good Luck and let us know how you got on
Geoff