Eating meat creates four times more greenhouse gases than being vegan, landmark study finds

There's an order-of-magnitude difference here. The worse case production of crops for human consumption comes out ahead even compared to best case production of animal products

Plant-based foods have a significantly smaller footprint on the environment than animal-based foods. Even the least sustainable vegetables and cereals cause less environmental harm than the lowest impact meat and dairy products [9].

https://www.mdpi.com/2072-6643/14/8/1614/htm

In terms of cost:

It found that in high-income countries:

• Vegan diets were the most affordable and reduced food costs by up to one third.

• Vegetarian diets were a close second.

• Flexitarian diets with low amounts of meat and dairy reduced costs by 14%.

• By contrast, pescatarian diets increased costs by up to 2%.

https://www.ox.ac.uk/news/2021-11-11-sustainable-eating-cheaper-and-healthier-oxford-study

In terms of biogas, here's a video looking at hog farming and talks about the problems with biogas at this point in the video:

https://youtu.be/WsUNylsiDH8?t=825