I recently bought the Exact Case for the Surface Pro 3 at an extremely low price: less than 3 dollars!.
The case qualify is not bad but is not top of the line either. I like the fact that it includes an elastic band as a handler in the back that will allows you to hold the Tablet second with one hand.
There an two minor problems with this case, the first one is the top of the case cover the vents and as result the SP3 can get hotter than normal. So I modified it cutting a hole in the case like it's shown in this picture.
That increased the air flow up to a decent level.
The second issue was the case leaving very little space in the right side to execute the gesture to call the right side charm. So I had to cut the case in that area leaving more room to do the gesture without accidentally touching the Windows button. The below picture shows how this change looks like.
Summarizing: with these modifications you can have a decent case for less than 10 dollars including shipping.