hello there jaime,
I have just recently bought 15 I.R. sensors for an installaition I am working on. I found two for around the $5-$6 range
http://www.dealextreme.com/details.dx/sku.7330
http://www.dealextreme.com/details.dx/sku.10082
i went with the first link because it is a rabbit, of course.
shouyld get them in a week or two.
i know a bit about hackingt them(not too hard to do it seems)
let me know how this project comes along as we might end up being a valuable resource to one another.