Although I've not leafletted in bulk, generally I've found that each leaflet dropped is eventually worth 50p - £1 per year. This is in smaller quantities though so the average results may be spurious. Sometimes work will grow very quickly in a small area once people see you working there. Other times there will be nothing.
For those who doubt my numbers (and I don't present them as being accurate on a larger scale), a 1% conversion rate would be 1 job for 100 leaflets. If the job is say, £15 x 8 times per year, that would be £120 per year. So, a bit over £1. Neighbours may join in once they see you around a few times. The flip side is that there may be one offs or that there may be no responses to 2 or 300 leaflets.
It can be quite a motivator thinking that each time you drop a leaflet, you've just gained 50p to £1 per year. Of course, you have to do the cleaning too
