| It would help to know where you live in order to answer your question better. Around where I live, the going rate per square foot is about 70 cents for two finish coats of paint. That rate will be a little more, if the walls or ceilings have any sort of texture on them. If there is extensive work to be done other than painting, such as washing of the walls, scraping, patching and sanding, then that will add to the cost. If you need to paint a coat of primer first, that will also add to the cost.
I always insist on applying two coats of paint. It is spelled out in my contract. There is no such thing as a one-coat paint. To advertise that there is a one-coat paint is just a way for a paint manufacturer to sell their paint. The only time that a one coat application would work is if you are using exactly the same colors on the surface to be painted and the last paint job done on those surfaces was within say, as years time.
If you have a contract (which I hope you will have, as this protects both you and the customer), you can make a provision in it that will allow for any additional work that is performed will be mutually agreed upon by the customer and the contractor. Also, in a contract, you can specify some sort of guarantee to the customer.
If you want to ask about contracts, I can help you with that. The answer would be lengthy. But, you have just wanted an answer as to how much to bid. I thought that I'd better touch on the contract thing though, because it is important. | 
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