There is no sense to applying any "rule of thumb" to a church job = except to make your best guess, then double it!
You can count on various quirks of the building - height, fixed seating, etc. - to make access a real challenge. You'll be spending money on lifts, scaffolding, etc.
As others have mentioned, religious institutions have earned a reputation for being bad customers ... constant pressure to cut your price, quibbling with committees over every detail, endless changes, and very slow payment.
