Loen, with posts like this it helps to give as much detail as you can, is it 15mts of one colour vinyl? Printed image or cut vinyl? Have you to produce artwork or is it supplied? If you have to do artwork do they have a complex logo that you need to reproduce? How much vinyl is on the vehicles already and what sort is it?
Like Rich has said removing the old stuff is the tricky bit so maybe offer an hourly rate, it may be that it comes off really easily so won’t take you long to do but if you quote say £90 expecting it to take you 3 or 4 hours and it ends up taking you a week because the old stuff just won’t come off or worse it is reflective then you are going to be out of pocket.