There are proofs that GMO varieties reduced the price of all the major crops that use them. There are many estimates of these price savings, which are quite consistent. A study in the prestigious Journal of Economic Perspectives by Barrows, Sexton and Zilberman (see below), estimates that on average the GMO contributed to 13 percent decline in the price of corn, 33 percent decline in the price of cotton and 34 percent decline in the price of soybean in 2010. These price reductions increased the availability of these feed products reduced the prices of meats, eggs and milk (as well as textiles). The impacts would have been much bigger if the use of GMO in other crops was allowed and if more countries would have adopted GMOs.
Barrows G, Sexton S, Zilberman D. Agricultural biotechnology: the promise and prospects of genetically modified crops. The Journal of Economic Perspectives. 2014 Jan 1;28(1):99-119 effects.