Kenneth's post is correct. I think the most you'll spend on cardio equipment should be on your shoes!
Having said that, I will still make a suggestion as to one other piece of equipment. A sled is great to have. But you don't have to buy a nicely welded, painted, fabricated one. I chopped a 4x8 sheet of 1/2" plywood down to 4x4. Then screwed some 2x4's onto the front of it in order to make a box in which the weighs would sit. I then screwed 2 J hooks into the front 2x4 to which I attach my rope. I also attached the seat of an old high chair to the back of the sled, such that my daughter can ride on the back while I pull. She loves it, and it's the best kind of daddy/daughter time I can think of. All tolled, I think that cost me about $10-$20.
But I wouldn't have done the above if not for my daughter. I would just stick to pushing/pulling my Jeep around the block. With gas what it is, I may just start pushing it to work too.