For example, by weight I eat almost only vegan food.
Nevertheless, I make 2 exceptions from 100% vegan food. I typically eat daily around 50 grams of chicken liver and one spoon of cod liver oil, which provide all the organic substances that are not available or available in too small quantities in vegetables, e.g. vitamin B12, vitamin D, EPA, DHA, choline, taurine.
There are vegan supplements for all those substances, but they would indeed cost more per day than all the food that I eat normally. Even if I could afford that, I would consider that as an irrational choice.
For example, non-animal sources of long-chain omega-3 fatty acids cost about 8 times more than fish oil (per fatty acid content). The cod liver oil costs me about $0.60 per day, so a vegan supplement would be about $4.80 per day. Where I live, in Europe, all the food that I eat in a day costs me normally no more than $5 per day, so already only one of the vegan supplements would cost as much as the current complete daily meals.
Moreover, previously I was making an extra exception and I was taking most of my protein intake from turkey breast or chicken breast.
With the proteins provided by food that was almost pure animal protein, there was a considerable freedom in choosing all the other food sources.
However, removing meat from the diet makes it extremely difficult to eat enough proteins without exceeding the daily energy intake for a sedentary life, unless you are willing to waste money by buying plant protein extracts that are much more expensive than meat.
While I have found a combination that provides enough proteins without providing too much calories, it does not have any degree of freedom, so removing meat from the diet has forced the introduction of a fixed part of the diet that cannot be easily changed from day to day, for more variety.