I would love to think that missing meat one day a week would make a difference, I was vegetarian for twenty years (16 -36yrs), now I eat organic meat from a local farmer. I still like my old vegetraian meals so we don't eat meat every single day anyway but I really think if people want to consider the environment and cost their meals take on it, they would need to make sure they don't buy meat that has made a huge journey before getting to the supermarket shelf.
As far as weight loss, vegetarian dishes are of course less fattening, people do eat too much meat daily.
And cost, well when I was really broke I could always stretch my pennies to buy more food as a vegetarian then I would have if I was eating meat. These days the organic meat we buy from the farmer is much cheaper then supermarket prices, but I think people often think it is the other way around and don't check these things out.