Note that the costs will run you as well. Because the print will be 18'' in width, and basically two feet long, (think of two one foot long subs stuck to each other at the ends,) all that space will require ink, as well as a special kind of quality wax paper to print it on, (like most posters.)
You also have to factor in the use of renting a printer from a company that can make use of paper that big, which will also cost you money.
Also, shipping expenses.
And taxes may apply on all the assets mentioned above.
Those costs don't really reflect the quality of the art, but the resources that are used to make it. Heck, a completely black print will still probably cost around $30'ish, if only because the entire thing will need to be covered in so much pure black ink. You'd be lucky to get half of what you are selling it for.