Assuming your insurance covers you to work in these premises I would book the job as a 4 hour job.
Regarding low prices, the church is one of the richest organisations in the world, charity or not! By all means submit a low price if it makes you feel good, who knows you may get a few brownie points when it matters if you are a believer

The 4 hours suggested, assumes that the church is reasonably clean to begin with, that no special operations are required, that no glass is required and that no special items are included in the clean, also that there are no problems with access to power etc.
Broomcklean, makes a valid point regarding other work that could well come out of this, but, if you go in low, you will be expected to maintain this discount regardless, also party cleanups etc etc should be priced accordingly.
Regards,
Rob