Sony makes one that has a 5 inch diagonal screen, uses HDMI cable, sells for $500 with all the parts. They say it will plug into any dslr- works just fine on the A7.
I was wondering about the component cost to a camera maker of a good quality EVF panels, to get an idea of how much an EVF adds to the retail price of a mirrorless camera. Two upper limits are the best of the external EVFs offered by Olympus and Sony, both of 1024x768 (XGA) resolution: the $279 Olympus VF-4 for Micro Four Thirds cameras (using the 0.48" Epson L3F04X-8x panel), and the $238 Sony FDA-EV1S for NEX cameras (which at a guess uses a 0.5" OLED panel from Sony.)
In comparison, there are entire low-end film SLRs which cost less than those external EVFs, so it could well be that the high-volume, low-end assemblies of mirror box plus penta-mirror VF add less to the cost of a camera than a good EVF does … for now.
Note that I am interested in the good quality panels suitable for an "SLR alternative", like the 0.48 inch XGA resolution Epson in the Olympus VF-4 and E-M1, not the smaller and lower resolution panels used in some super-zoom compacts, so looking at the lowest price for a compact with an EVF is not helpful.
P. S. If 800x600 (SVGA) as in the Olympus E-M5 is acceptable, a lower price is $160 for a Panasonic accessory EVF. Still almost as much as an entire Vivitar film SLR!