To give you an idea of what the performance value of a GPU is I’ve gathered benchmark results and put them in the table presented on this page.
You’ll see “gaming cards” perform very well, the “pro cards” are less capable if you compare “result per buck”.
The GPU names link to entries on TechPowerUp’s GPU database.
If you want to compare GPU’s to get an idea of the difference you can use GPUBoss for instance.
For instance, you’ll find a comparison of Nvidia GTX1070 vs AMD R9 290X on this page.