Veganism is the practice of abstaining from the use of animal products,
particularly in diet, and an associated philosophy that rejects the
commodity status of animals. An individual who follows the diet or
philosophy is known as a vegan. Distinctions may be made between several
categories of veganism...
Learn more about Veganism on Wikipedia
Protein is of course also found in plant-based foods, so that the requirement can be easily met with a varied vegan diet: Good sources of vegetable protein are, for example, pulses, nuts and wholemeal products.
On the contrary: the "basics" of a plant-based diet such as potatoes, rice, pasta, pulses, fruit and vegetables are even among the cheapest foods of all. If you wield the wooden spoon yourself and do without superfluous extras such as so-called "superfoods" and vegan convenience products, you can eat a healthy and balanced vegan diet even on a small budget.
This is a so-called naturalistic fallacy, because the "naturalness" of a thing says nothing about whether it is good or bad. In this sense, one could also argue that vaccinations, clothing, houses, appendectomies and electric light are bad because they are "unnatural" - which certainly only very few people would subscribe to.