What type of signal does a display's logic board output?

What type of signal does a display's logic board output?

I'd like to build a cheap VGA capture device, since the cheapest one
available on the market, the Epiphan VGA2USB, is $300 MSRP. Could I use a
logic board on a small display to accomplish this? Soldering and such
aside (I'll probably hook this up via RasPi GPIO), do the boards
themselves convert the VGA signal into a software-decodable format (R on
one pin, G on another, etc), or do they output instructions specific to
the display? What makes a VGA capture device so much more expensive than a
composite one?
Also, I'm aware this might not be the best place to ask this, but I
couldn't find any other stack exchange sites that seemed more appropriate.
If you know of one, I'd like to know it- leave a comment!