Not everyone's homes. Here in Europe there's a GMO ban. Just think of how much less roundup is used when roundup ready crops aren't planted.
Probably about half as much. Even without the GMO RoundUp Ready crops you could clear a field with the stuff and then plant corn in behind that in 7 days. Saves on tilling costs and also better for the environment as you're not disturbing soil. With the GMO stuff you might get another spray 4-5 weeks into the crop of RoundUp to kill new weeds but anything beyond that the corn itself will crowd out the weeds.