Does it cost a lot extra in insurance for a van carrying a WFP system
a little more yes, the problem is finding insurance companies to quote you, you will not be able to use comparison sites due to the fact you have wfp system installed.
Here are a few that will quote
A- plan
Autonet
gleaming insurance
Alexander Swan
There are others but have slipped my mind.
I pay about £700 give or take per van with 2 drivers on each policy, hope that helps a little
If you use A plan 01993 841 841 mention Window washers and ask to speak to Dan he deal with this all the time

This year A- Plan are the cheapest I have found, they also give avan and system if your in an accident which is good.