Choc milk is good. Or after a longer ride, protein powder from Boots, £9 per 250g with 80% protein content. Mix 2 big spoons with Nesquik and 750ml milk. Prob 30-40p a go. But no need if your muscles aren’t hurting after a ride. (edit, just looked up myprotein, will give that a go as it’s a lot cheaper)
Otherwise use dried milk powder in place of protein powder, really cheap. 30% protein so 3 tablespoons is about 10g protein. Adds a bit to the milk anyway.