I used to do an awful lot of initial cleans for builders.
By and large you charge three times what you would normally charge per window.
Check how quickly they will pay you though, and if it is a case of invoicing and waiting for it to go through the various channels, then charge at least 4 times more per window than you would normally charge.
Point out that if there is compo (sand & cement) on the glass, then the glass may well be scratched, ditto the frames.
If the frames are UPVC and you have to take off that blue protective film....then run a mile, the stuff is an absolute bloody nightmare, and I personally won't touch those jobs with a bargepole.
This stuff rarely peels off easily, often you are picking little bits no bigger than your fingernail.
If you give them a price, make sure you give them a separate price for the different style houses that are on the site, they won't all be identical.
Also insist on a minimum period of notice, if they want it done 'today!' Then make sure that they are aware that you will have to charge a call out fee on top of what you have already priced.
And if they go beyond the payment schedual and are slow paying, whatever you do, don't keep on cleaning fresh houses!!!
Have fun, you can make good money, but you have to watch builders
regards,
Ian