When I bought my 2007 Jeep Liberty it did not have cross members for my roof rails. I needed these because I haul a lot of wood for other projects, but wasn't wanting to pay $50 for the rails. You should be able to do this for $30 or less, I did it for less than $5.
Step 1: Measure
Luckily the Jeep has holes already in the roof rails which makes it easy to just add the rails I'm making attach without altering the Jeep one bit. I measured from hole to hole to know where to drill holes in the cross-members. The front is narrower than the back.
Step 2: Cut Down Cross-Members
I had these 1" square aluminum tubes from a scrapyard that where given to me for free. I did find some online for $10 for 48" length if I needed to. They are 60" each so I cut them down to 48" so I didn't have too much over hang on the sides but enough so if I wanted to attach a tie-down strap to it.
Step 3: Measure and Drill Holes
I measured the holes on the roof rails the front set are 39 3/4" and the rear are 41 3/4" apart. I marked this out or the rails and drilled these out with a 1/4" bit.
Step 4: Tubing Caps
I found some rail caps at the hardware store to keep bugs and debris out.
Step 5: Install Caps
I used a rubber mallet to tap in the caps so I didn't damage them. Make sure you do this after drilling out the holes as to not block in the shavings from drilling in side the rails.
Step 6: J-Hooks
I found j-hooks which made it easy to just hook them into the holes on the roof rail. I cut them down so there was just an 1 1/2" of the thread left. Make sure to cover the bare metal if you have to cut down your bolts, i just shot them with a bit of spray paint once I was done.
Step 7: Bolt It Together
Attached the tubing to the roof rails with the j-hooks, an over-sized washer and nut to finish it up.
Step 8: All Done
There you have it, a pretty simple project.