I could only find one copy of your book on biblio.com. Here is the description. It was priced at $270.
1785 Full Leather Front board missing, spine intact, text very good, clean. (723) pp. Plus Index and List of Plates, frontis, 55 ful page copper plates with 5 maps and plans. 10" x 14.
If yours is in better condition, you could contact an auction house. A good place to look is on www.americanaexchange.com.
www.pbagalleries.com has a form on their site where you can inquire about the value of your book.
