Mural Painting costing

Hello all I'm a newbie here so please accept my apologies in advance if this is the wrong place to post this question, or if someone else has already done so. I've been commissioned to paint a large indoor wall (about 29 square metre) with a detailed, many colour 'panoramic vista', for want of a better expression. The design is all done (on a roll of wallpaper) and now I have to cost up the main work. I have absolutely no idea how long it will take me (it will be my first big scale work) or how much paint it will use (I intend to use acrylics). Is there any reliable website which could give me an idea of the kind of price per hour I should be charging? And how much paint I'd need? I need to be able to show the person commissioning me that I'm basing my costing on 'going rates'. Because of my lack of experience in this field I'm concerned that if I pitch it too low I'll end up severely out of pocket if it takes me longer than I've bargained for, or if I use significantly more paint than I thought I'd need. On the flip side, I don't want to over-charge. If there's any info or advice that any of you could give me (or even point me to another post where this kind of thing has already been discussed) I'd be really grateful. x