Long story short, my wife needed a treadmill desk, and we don't have the resources to buy one (they run hundreds of dollars). As such, I built one and thought others might like to see.

Step 1: Gather Tools and Materials

Materials:
3 or 4x: 2"x4"x8'
1 or 2x: 1"x2"x8'
1x: desk surface (1'x4'x1")
2x: hinges
3" screws
Polyeurethane
Sand paper
Paint brush

These should all total about $50.

Tools:
Jigsaw (any other kind of saw should do, a jigsaw is all I have)
Drill
Tape measure

Square

Recommended:
Hearing protection (more of a must if your saw is as loud as mine)
Countersink bit (I use this bit to make guide holes to prevent splitting and countersink the hole at the same time)

Step 2: Make Your Cuts

Measure twice and cut once! (My two base pieces are 2" shorter than intended because I miss-cut one of them) Also, if in doubt, cut long. It's easy to cut more off later. I bought more boards than I needed just in case I did mess up.
The boards you should have at the end are as follows:
In 2x4's
-4x 42" legs (shorter or longer depending on how tall you and your treadmill are)
-4x 12" crossbeams
-2x 24" feet beams (I accidentally ended up with 22" feet because I cut one a bit short)
-2x 12" braces with beveled ends

In 1x2
-One length to go between the two legs of your treadmill, so a bit wider than the overall with of your treadmill. (I waited until my legs were in place around my treadmill before I cut this)
-One length the width of your desk surface.

Step 3: Assemble the Legs

To do so, I took the crossbeam, laid it on the ground, skinny side down, and laid two legs butted up against it and screwed them in, making a tall skinny U-shape.
Next, I adjusted the legs so that there was a 12" gap at the bottom, matching the 12" crossbeam at the top. (Place a crossbeam at the bottom if you don't care to measure) I had the foot beam centered so that there were equal lengths projecting from the front and back legs.
Lastly I added the remaining crossbeam in the opposite direction of the first crossbeam to keep the frame square, screwed in from both the legs and from the top crossbeam.

Step 4: Prepare the Desk Surface

I bought a pre-made desktop that was 11"x48" then I took a 48" length of 1x2 and screwed it on to make a pen lip. I had the 1x2 a little bit raised so that it wouldn't interfere with the movement of the hinges.

I sanded then varnished the top to give it a nice feel. I used wipe-on polyurethane. I can't recommend it, but you may find you like it. I left the rest of the desk unvarnished, but would've sanded and varnished it too if I had the time.

Step 5: Final Assembly

Place the legs where you want them around the treadmill. Cut the 1x2 crossbeam to length (keeping in mind it's easy to make things shorter later) and attach it to the desk legs, making sure that you aren't impeding your own movement when walking on the treadmill. I put mine on the backside of the desk, behind the treadmill. Then attach the two braces to keep the desk from shifting side to side.
Next, attach the desk top to the legs with a pair of door hinges, or really any hinge that you can screw in. It would have been much easier if I had someone to hold it for me while I attached it. Instead, I got to mark where the screws should go on one side, drilled guide holes, attached that side, then went to the other side and marked, drilled, and fastened.
Now my desk is nearly fully assembled. The frame is strong enough to hold the desktop and whatever books I put on it. It's entirely separate from the treadmill, except that the top of the desk surface is resting on the treadmills control panel. I cut two matching pieces and attached them to the top of the legs so that the desktop rests on the two 1x2 pieces instead of the treadmills panel.

I may revisit this to add a way to adjust the pitch of the desk, but its at a decent angle for now.