Here up north, veggies/things that grow underground and survive the weather and transportation without much effort are some of the cheapest stuff you can eat. So potatoes, carrots, beets, parsnip... i usually go nuts on carrots and beets if they have a low price special on local produce.
As for meat, while I do eat it I consider red meat to be a luxury product. I don't buy meat that's not krav (high demand eco label) marked except for a few exceptions carefully chosen after a bit of research. Only time i consume anything lesser is if a family member cooks it for me or if i eat at a restaurant (however in that case i usually pick fish). Restaurants are a bit behind so i prefer to motivate positive behavior so they realize there's consumer demand for animal and environment friendly products.
Because of these choices, red meat gets very expensive for me. Being a low income artist, this means I rarely consume red meat.
I eat quite a bit of fish, however I aim for MSC fish whenever possible or alaska pollock which is cheap and not at risk of extinction anytime soon.
When in a pinch/low on money i eat mostly vegetarian. Quorn and soy products are cheaper than meat if you know which products to look for! And it sure as hell is healthier than instant noodles.
As for eggs, the cheapest alternative to positive egg production here is ones that are from a local farm where the chickens are fed food that's been produced closely to the farm and they are kept in small enough packs that they dont hack at each other. In fact, these local eggs are the same price range (or cheaper if you take their larger size into account) as the standard eggs.
What's more environment and animal friendly and what's more economical will be different in each country. Usually local produce in smaller scale production will be cheaper and better for you because of lower transportation costs and less shitty handling methods during production. But there are definitely a lot of big exceptions to that rule. A few moments of research or asking fellow locals should lead you to the answers you need if you are interested.
I won't judge someone for eating a lot of meat. Especially not if it's what's cheapest in their location. People are only people at the end of the day, and their choices should be up to them.
Some random advice if you have vegetarian friends who eat dairy and you're gonna make pizza: ask your veggie friend about their stance on cheese. High chances are they are against animalic rennet, meaning you should have a quick look to make sure the cheese you choose has vegetabilic rennet.
Same goes if you're gonna offer them candy. Thinking of polka on christmas? Might not fare so well with your veggie buddy because of the red dye.
I can honestly say that knowing the vegetarian and vegan diet has been helpful for me. It is useful in case I ever become a waiter again (my understanding of their diet and ability to recommend things on the menu by it was very appreciated among the vegetarian customers, raising their impression of the restaurant's customer service) and it's also good to know so I can be respectful towards friends that live with that diet.