we dont price by the amount of vinyl used, we price by amount of letters at whatever hieght….
e.g.
we do lots of taxi names on the doors.. you know the type. drivers name and cab number at 1 inch high…  about 30 letters… ill charge £8
take the vinyl used and multiply that to fit into a metre square… ? thats allot of money for a few quids worth of vinyl…
what i am getting at is.. the van could have lots of lines of small text. and complex logos.. running at weird angles on the van.
you need to account for:
weeding
taping
design
cutting
fitting labour
and of course materials.
you shouldnt have a bog standard pricing… (not saying you have) just never say a basic transit will cost you £100 because your basic idea and the cutomers basic is never the same. the customer will milk it, if this is the case. 
we charge something like 35p for a 1 inch letter £1 for a 4inch and so on..
set yourself a pricing structure for text. 1inch upto say… 24inch..
now once your customer is sitting and you finalise a design.. you multiply each line fo text by its hieght/cost.  the customer sees you refering to your price structure and you come out with… “it works out at £180 as you can see” (inside you know there are lots of text but no big deail so…) you say.. if you leave a deposit or pay now ill do it for £150. he sees you have just knocked £30 off and knows or thinks? hes getting a good deal 😉 
this is just the way we have worked pricing over the years and works very well for us.. others may have a different opinion.