Without pics it's hard but we do a few car showrooms and with wfp 76 panes can be shot through really quickly. I reckon, without seeing it remember, that £50 - £60 would be acceptable to them and easy for you to earn in a couple of hours.
Insides will be different as you can't use wfp obviously. So, faced with the same decisions as you I'd Quote: Outside only ..... £55, Inside and outside ....... £130.
There you go; now watch as you'll get prices from £20 in and out to £200 outside only. I love this job
I would just add that metal frames can be a nightmare or can be totally fine !! Be prepared to have to re-rinse the windows again.
With that in mind, if you think they'll come up fine then £50-60 sounds fair, although I would imagine you'd be able to do that in 1hr30 or less ??
inside is a different story. How easy are the windows to get to ? will there be cars directly where the ideal place for the ladder will be ?? will the ladder need footing ?? (although there's some great ladder mats out there).
15ft . . is inside over 2 floors or are the windows really high ??
All problems that could add time to the job.
good luck