a professionally made one can be $1000, so I figured it'd be a good round number for an estimate.  You can find them regularly for $500 or less, depending on location and such, and building one should be about that much.  I didn't want someone to come in with a higher price and kill my point.

You wouldn't cut anything off, not that I would anyway, most wouldn't have too much extra stuff on them to take up space and they're designed that way so cutting anything would, like you said, weaken the structure.

besides, a portable table might be of use.

I was actually thinking the triangular gantry from the original torch table would be the kind of design to use.  It only requires one side to be straight and true, reducing the complexity needed to make a machine this way.  While the sides of the truck are probably stiff enough, they are probably not as straight and true as you'd like for a CNC table.  This means that the rails will have to be tweaked as you install them to get them straight enough to give your machine reasonable accuracy.  Driving both sides isn't hard, but getting two rails six feet apart straight and parallel for the nine feet or so you have on a truck bed would be a trick.  Not trying to overblow the problem, but just having to true one side saves a good bit of effort, and all you need on the other side is to make sure the wheels stay on the rail.

It would cut down your Y axis a bit.

Basically.  The two trucks on the one side make the system square.  This means that as long as that rail is true, the system should be true.  The other side needs to be locked into its rail to allow downward pressure, but isn't required to be set to the same accuracy standards as the other.  Biggest deal would be up and down, to make sure it's not pulling the tip off the depth you want.

I suppose that might be enough to make it worthwhile to just do it all the way, but if not it might save some trouble.