Have you considered buying used? I don't know if you can find a store that sells used racks or not, but it's worth looking. You probably won't find the feet, but the cross bars you should be able to pick up for cheap.
I would think that for a bag, you want cross bars of some sort, just to give support so it doesn't rest on the roof of the car.
Note that if you're feeling adventurous and know how to tie knots, you could go with the ghetto cross bars - a 2x4 from home depot cut in half so you have 2 4ft sections. Some rope to hold it down and you're good to go. Don't use crappy twine to hold it together. You should be able to rig something for $10 - if it's a one-time think so you can move, it might be worth it.
If you want to spend the big bucks, you can probably even rig a clamping system using some metal brace and lag bolts. I have a vision in my head but it's really hard to explain in text. Imagine the 2x4 going across the car, over the roof rails. You need something to clamp them against the roof rail so they don't go flopping around. Get a straight metal bracket (in the fasteners isle, for connecting lumber together), you want a piece at least 4" long. Get a few lag bolts, 3" should do. Drill a hole on both ends of the metal brace, space it so the middle of the brace goes under the roof rail where the 2x4 is above, and mark the holes for the lag bolts. Take an old sock and wrap it around the metal so it doesn't nick the bottom of the roof rail. Pre-drill pilot holes for the lag bolts, and instlal the inboard one through the metal brace. Don't thighten it snug, but make it so the distance between the 2x4 and the metal brace is the same as the height of the roof rail. Swing the metal braces perpendicular to the 2x4 and place the contraption on car, swing the braces under the roof rails, make sure the sock padding is in place and covers the edges, and install second lag bolt. Tighten lag bolts until the bar is nice and tight and won't move when you push/pull on it.