Agreed. You'll be hard pressed to get any kind of performance increase from those resistors, and if you really want to use resistors, order them from Digi-Key or an electronics warehouse. You'll be able to get the same resistor for a dollar or less.
Instead of monitoring and modifying the signal sent to the PCM, these resistors just change the static value of the sensor they're in line with. For example, you might put one in the throttle position sensor line to tell the PCM that the throttle is farther open than it really is. The PCM might change fuel curves to figure on a wider throttle opening, but the O2 sensor is still going to determine rich or lean, the PCM will adjust accordingly, and you'll gain nothing. Just push the throttle down more. Same result. As for warranty risks, if you take the car in with engine driveability symptoms and they find you've put in one of these "chips", they can very easily tell you to pay up or go home. If your rear end dies or your brake system fails... that's not the computer's fault, but anything related to the PCM could then be considered suspect and you could have to pay.
The expensive chips monitor most (if not all) signals and modify things accordingly to actually help you gain some power. They also don't do a whole lot on a stock engine - believe me, if Ford thought you could get a bunch more power out of the car with a simple program change and not harm anything, they probably would have. There are a few exceptions, but for the most part, the stock program works very good, if not ideally, with the stock drivetrain.
Do the mods that will make your car feel better to you. As Mike suggested, rear end gears will increase the off-the-line performance by allowing the engine to get to its peak power faster and stay in the strong part of the powerband. Subframe connectors make the car chassis much stiffer, taking up less power in body flex and giving you stability in the turns. Same with suspension modifications - they'll make the car feel better to drive, and probably feel faster. If it's a stick, put a performance shifter in - it'll feel like a whole new car.
My theory is, if it were free or that cheap/easy, it would have been done already...
-Mike