Pricing Murals

Oh, no! It's a how-much-do-I-charge question. I do have a sort of idea, but I want to run it past someone who might have an inkling so that I might discover whether I'm barking up the right tree. My first mural - on the school - was done for nowt. This one is a commercial property and I'm charging. Something. But I can hardly charge top dollar for my second ever mural. I decided that it might be simplest to have an area-based formula with add-ons if necessary. Being a former scientist, I want to use SI units - so that's metric square metres (m^2), not imperial square feet. I've estimated the size of the first phase to be 9 m^2. (I'll get this measured) If the paint covers 10m^2 / litre (figure stolen from DIY site re. emulsion and rounded down), the cost of paint is (at £20 / m^2) £200. I've estimated that it will take 7 short days or less to paint this. (By short day, I mean "during school hours") If I charge £100 per square metre, I get £100 an estimated day. (I used to earn £30 an hour, so that seems low, but the painting days are only 5 hours long so - while still low at ~£20/hr - its not as bad as it might be) Square metres are quite big. I would sell a painting that size for well over £100... but this isn't a movable painting. It won't need the same level of refinement. Phew! Bottom line - £100 per square metre*. Stupidly low? About right (second ever mural scheme, remember)? Waaaay too expensive? If I add some on for prep work (research and sketches) and write down what I will and will not do for that price (ie, I don't prep walls, varnishing is extra and the client is welcome to do it themselves) and lay out payment schemes (50% now, 50% on completion of phase), I might be covered... ==== *One square metre is approximately 9 square feet. Well, it's more than that, but near enough.