Simon, James44 is correct. Any waste water produced must be collected and discharged at an approved designated local authority disposal point.... you cannot simply empty the foul water out willynilly ie at the roadside/garden/carpark.
If you could comply with all the guidelines, rules & regs then the average price for doing a 1100 litre 4 wheeled bin is £13 - £18 per bin (around my area anyway) and expect to spend up to 20 mins per bin on the first clean and 10 mins thereafter obviously depending how good a job you wish to do on them.
Dave.