The Venus is one of the costlier options at US$389. BUT -- you also get 5 USB ports, an Ethernet port and space for an SSD inside the enclosure with a SATA interface. It's basically an eGPU *and* a Thunderbolt dock in one. And it can provide a little extra juice to charge a laptop.
You can go cheaper -- I think the Sonnet Breakaway Box (350W) is around US$270, give or take, but doesn't have the additional ports. In reality the additional ports aren't too useful in TB1 since you don't have too much extra bandwidth to spare and give away to other I/O. But I'd imagine they are sweet in TB3.
On the relatively low end, an eGPU enclosure and card would start at around US$400, with something like the 350W Sonnet Breakaway Box and a GTX 1050 Ti. (And if you don't plan to upgrade to a new machine any time soon, it's probably all you need with a TB1 system.) Of course, you take the risk of dealing with an unsupported environment in the NVIDIA cards with High Sierra right now. But you could also pick up something like an RX470 or RX480 now that the cryptocurrency craze is starting to throttle down and demand for the Radeon cards is bringing prices closer to reality.
For what it's worth, the Open GL based benchmarks I've run are giving me 3-4 times the FPS with the eGPU as the internal graphics.
