The US is pretty much screwed for prices in most respects for prices on food, why? Because of de-regulating measures to control quality and the fact that the US owes 1 trillion dollars to China. So... in light of costs, yes, being a vegetarian in the US can be expensive, but no more than buying meat.
I don't know why anyone would charge 4 dollars for a head of lettuce, but yes, these are prices we are seeing. I also don't know what will happen with the fact that China has announced it's going to be devaluing US currency by 30% so expect prices to go up even more.
Unfortunately, the culture of America is a bit unprepared for these kinds of changes, and it is already detrimentally impacting lives of families who now eat fast food as the "cheapest means to eat"... so don't expect vegetarianism to skyrocket any time soon in the US.
All that being said, being vegetarian in the US is still cheaper than being a meat eater by far. Instead of meat, substitute beans, and protein based foods that aren't meat-based. You might have to do a little research. However, being a vegetarian and wanting to still eat meat-like products, yes you will start paying a lot for your diet.
I am a part-time meat eater. I am not fond of meat, but live in a country that consumes a fair amount of meat. In a sense, I guess, I'm a whatever is offered to me eater, but if I have a choice, I go for green. Not that I'm especially proud of this either. In order for me to eat vegetables, do you know how many millions of animals perished?
Basically, my attitude is this. Consume less. Need less. Learn to subsist on just what's needed. Refrain from all products that cost much life. Meat or no meat, something must die for you to live. Unless you grow your own, in which case you get to do the killing yourself to grow your food. You cannot entirely avoid costing life on this planet so long as you live in mass-production based civilization. Know this and reduce a bit at a time what you need. Give more, need less. That's what I'm doing.
There is no further morality in eating rice that killed millions of animals, insects, rodents, (you name it) for the rice paddies, than eating a cow where millions of insects, animals also died in the caretaking process of the animal.
Mass production costs life. This is the bottom line. Can you single handedly stop it? Probably not overnight. And not by making anyone else do anything, by becoming aware of the fact that for you to live, other living beings die. Never forget, and always dedicate merit to that which lived for you to continue your existence.
I am fortunate to live in a poorer country now where food costs less, like close to the price of production, and there is still a lot of local growing here. Being vegetarian isn't easy anywhere, because agriculture has its own HIGH toll in lives too because of mass-production. Buy local if you can, farmers markets and such. Countless beings that hope and fear for their survival and therefore are sentient, lose their lives to our consumption.
So, while I always encourage vegetarianism, I also suggest that it's best to simply need and consume less, have the lightest footprint you can have on the world. The point of liberation is for ALL sentient beings. If I understand that correctly, the Buddha did not openly say, "BE vegetarian" because he also realized, hey, it's not only meat that costs life. So it seems he put more emphasis on the doing no harm, that is to take less and give more. To transform yourself into a source of life, not a drain on the world's resources. This is far deeper, I think, than just being vegetarian. But I've never been a fan of rabidly being pro or against either, because the truth of agricultural industry is just as grimy as the truth of meat-production because it is based on greed.
My stance isn't too often that popular as I'm not an outright hardcore vegetarian. When I first started investigating, I wanted to be vegetarian to protect life. When I realized it's not eating meat that outright feeds the entire machine of consumerism, my stance had to change. Consume less. Agriculture exterminates a lot of life too. Consume what you need and no more. It's a kind of altruism that doesn't just encompass not eating meat.
Then again, I could be wrong, I'm have not been a Buddhist long, 2 years. So I am often mistaken, and gladly corrected. These were just my findings from my own investigation.