S.H.
Its not in your price range, but when we had a house with almost white carpet and 4 kids, this was a life saver. It is just like the ones you rent in the stores and it works so so well. This probably won't be what you want for spot cleaning, but if you do you entire house/rooms often...this one is amazing!
http://www.kohls.com/product/prd-776882/rug-doctor-x3-val...
Don't even think about getting it without the 30% off coupon though that kohls almost always has :) But with the coupon, you're looking at about $300.
ETA: I don't think carpet cleaners and lightweight go together. They need to have water in them...so that makes it heavy...and I think some weight helps with the cleaning/suction. It is a workout no matter what the machine. The above one is super easy to clean and load with water though!