250 for a single print seems pretty high. depends on how established the artist is and career artists typically have strong opinions on this stuff (i don't know anything about this artist but have some artist friends who have interesting and varied views on the value of their work and creative work in general).

i'm surprised toyota didn't purchase the rights exclusively when they contracted the artist to do the work, and that this artist is even in a position to be selling prints.

you might be able to print those 600px ones as-is as 5x7's and line em up side by side inside one of those wide picture frames. i bet you could get away with that especially where the artistry itself is very soft looking and if you don't push too hard will be forgiving if you blow it up a little. i think that would look really cool.