You have a couple different choices of sensing technologies; which one you pick will determine what power you'll need in your processor board.
If you want to go with full customizable vision, you'll need something on the level of the RoBaord that [PAR_WIG1350] pointed out. I would personally point you toward the Surveyor
Blackfin board (
$195), since it already has the camera integrated. It's programmable in C; I found it pretty easy to learn the API they've created. They've also come out with Linux support recently if that's easier for you. You can get it with Wifi for $350 if you want to have it report back to the mothership...
For slightly reduced functionality, you might want to check to see if your team has any of the CMUCam2 board from 2005/2006/2007. We only used the color blob tracking function for FIRST, but they also have a simple motion detection/frame differentiating function that can be run. You could pair one of these with a low-powered microcontroller (see below) that would control the servos.
Taking a different approach, you might could use some sort of proximity sensor. Options include
PIR sensors (what they use in automatic doors), or ultrasonic or infrared rangers. You should be able to get any of these for less than $25. If you only want a proximity alarm, you should only need one-three (depending on the desired monitoring angle and sensor type). You may have to go to a higher number if you want to actually aim accurately. You'll have to be careful if you attempt this with the range finders, as they rely on sensing reflections, and crosstalk between the sensors can invalidate the readings.
These types of sensors would only require a fairly low-power processor. The common choice would be an Arduino (as is the case in the linked PIR sensor tutorial), but if you want to go
really low cost, TI just came out with a
new controller board that costs less than $5 that looks interesting.
--Ryan