This plastic bottle cage weighs only 25g, saving up to 34g over a generic alloy cage, or 68g per pair. However, purchasing these can be a gamble, I have has different cages weigh in anywhere from 25g to 35g, depending on the vendor. You may have to purchase pairs of cages from multiple vendors. The cages used to weigh 35g, but apparently in order to reduce costs, they have been lightened to 25g, and are flimsier as a result, but still functional. These cages have a tendency to make clicking noises if they are not holding any bottles however. Unlike the aluminum cage, it will not mark up your bottles as much either. At $1 each, or $2 for two, and saving 17g per dollar, it represents an extraordinary value, very low cost upgrade. It can be purchased on eBay.
Price: $2/pair ($1 each) [Very Low Cost]
Value: 17g/$ ($0.06/g) [Exceptional Value]
Recommended with caveats
When empty, these cages tend to be noisy, and it may require multiple purchases to get the low weight ones. They still represent an exceptional value though, even if you have to buy multiple, or end up with 35g cages. You may want to check out Generic Plastic Cage #2 instead, which I prefer.