http://barefeats.com/ which in the Apple community is known for graphics testing of Apple products and other solutions has done a lot of work with external GPU enclosures.

One vendor (Sonnet) [1] has a 580 solution at 499. B&H has the no GPU unit at 299. So there are options.

[1]https://www.sonnetstore.com/collections/egpu-expansion-syste...