It really depends on the number of unique tiles (which, in turn, depends on the modularity of your buildings, the number of different styles of buildings, the number of different ground surfaces), the size of the tiles, the scale (larger scales mean more tiles are needed to depict the same type of object, and your examples have different scales), and whether the artist will be working from established designs or doing their own design work. And of course, different artists may charge differently for the same style and amount of work due to their working speeds or circumstances. The style and how well it matches the artist's skills will also be a factor, even in the case of those artists who charge flat fees.
Something of the scope of any one of those images would probably cost you in the low hundreds of USD. It's hard to be more specific than that, because it really depends on your exact needs and the artist you find.
