1080 GT is around $380
1080 TI is between $475-$525 used.
RTX 2070 $700+... Ouch
Currently, it's not worth it even after 3 months all games that have implementer RTX show heavy FPS hits with that feature on to a point where most turn it off. Without that feature, a 2070 just doesn't perform strong enough to out value a used 1080 TI. Just like hairworks, this is another unique poorly implemented option that will get turned off in most situations.
Google up "ray tracing fps issues," notice no updates since August.