Pricing for mural sized b&w print

Good day,

I have received an offer to sell a mural sized print of one of my images that I normally print in 11x14 from a 4x5 negative. The requested mural would be substantially larger, that is ca. 7 by 13 feet. I am sure someone has been in a similar situation. Would you care to contact me via email and provide some guidance as to how you would go about scaling up the sales price?

Thanks very much in advance,

Markus Albertz