builders clean's can be good earners, but as for pricing its diffucult, i price only once a site has been inspected by me, and it depends on how deep the dust is and how much plaster there is on skirtings, and paint splashes, and lables on sinks baths and loo's, and size of kitchen as all these are time consuming. Materials are not a lot, a good hoover that has a good filtration system ( we use dyson cylinder's for builders houses) mop,buckets, good cotton cloths. multi purpose, loo cleaner, and blade scraper and plastic scraper, scourere'sand floor gel, stainless steel polish and glass cloths. And staff that enjoy builders cleans.
The way I judge a good job done is, when finished walk off site for a fAg, go back as a prospective buyer, does the bathroom look clean and new (no water marks, dust ect) Sorry flat mop for tiles (forgot that one) Kitchen cupboards tops too and inside, do they look new? by now you should like and feel clean in the building. But you as a good cleaner will find things that the builder will need to add to his snag list, so do tell them, as it makes you look more professional.
As for payment you alway give a prospective customer your terms of payment in writing, but the odd ones can take as long as 60 days.
Hope this helps.
As for approaching, can't help with that as all our builders clean's come via word of mouth as does most of our work, we get the odd call, on average 1 per month from the yellow pages or thomson, but most of them are domestic's.