Great video!

It looks like you're comparing dry mass of grains to wet mass of meat in the video. I assume masses are based on wet weight for both vegetables and meat? I think it's important to consider that while the cost per 100 g might be lower for vegan sources, the sheer amount of the product you'd need to consume to obtain the same amount of protein as meat sources is much higher.

For example, eating 1000 g of spinach for 20 g of protein makes it an incredibly inefficient and effectively impossible source of protein. Aiming for 100 g of protein per day, you'd need to eat 5 kg of spinach, which would be both unpleasant and difficult for most! That one doesn't work out as a great value either, as this would cost you 5.45€.

In comparison, wheat flour is very cheap, but since it comes as a dry good you'd need to add at least 50% of the mass of flour as water to make it consumable (e.g. as bread). That effectively reduces the amount of protein (and calories) consumed by 33%. Still a good value, but you'd need to consume 1363 g of bread to meet that need. Reasonable. However, that would amount to 3420 kCal of calories to consume 100 g of protein, far above the average daily caloric need for most. Additionally, that would mean you'd be missing out on many nutrients necessary from other foods.

Split peas certainly come out better. Most recipes seem to call for a 1.5:1 ratio of water to peas by volume, which works out to 2.8:1 ratio by mass. Assuming some loss to evaporation, let's say maybe 2:1. For 100 g of protein, you'd then need to eat ~1000 g of wet peas. Doable, though that's a lot of peas! Better be prepared for that much fibre. That adds up to 1955 kCal, which is roughly the daily caloric need for the average person. Doesn't leave a lot of room for additional stuff. If you reduce the amount of peas to accommodate other things, you will need to lower your protein intake. Which is fine for most people, where 60-80 g of protein is around the target intake amount.

Sounds like peas are a real winner for the balance of protein and calories. I suppose that's why they're such a popular food item globally, especially since they can be grown in quantity and are easy to dry, store, and prepare.