My understanding (and I could be wrong since neither own nor have ever used either of these things) is that you could indeed buy a Jpac and use a 'cheap VGA card'.
But if you go that route, you must also find some way to get that 'cheap VGA card' to output 15kHz. There are numerous software means of forcing your 'cheap VGA card' to do this but I also believe that not all 'cheap VGA cards' are even capable. So choose your 'cheap VGA card' wisely.
The way I understand it, the JPac provides you with 4 things: (1) a way to hook up your Jamma harnessed controls (2) a 'protection' against sending "bad" refresh rates to your arcade monitor (3) an interface between your arcade monitor and your PCs video card and (4) a power bump to bring the video signal up to what the arcade monitor wants. The ArcadeVGA card provides you with an easy way to output your PC's video signal at the specs that your arcade monitor wants.
If I'm wrong or if I missed anything, I'm sure someone else will chime in .
HTH