Yeah, protein powder is key, but overall, not cheap.
A few years ago, before covid and brexit etc, you could easily get kg pure whey for £50... now proteinworks and myprotein are selling these even after discount for £100... it's crazy for what it is!
Look on ebay, it's a bit cheaper. Around £60 for 5 kg, although usually not the best flavours, or if they are, loaded with carbs. I'd imagine you're looking for pure whey, read the label :).
Also, if you read the literature, you probably don't need as much as you think. Media plugs the 2 g/kg rule, but in reality it is about 1.6, and that is for elite athletes, most people can get away with 1, as long as it is spread out through the day every three hours or so, with the main bulk after working out.
I like my coffee, so one in the morning (5 g), shake a few hours later (20 g), lunch with chicken/sandwich meat before working out (20 g), shake after workout (20 g), normal dinner (20 g), casein before bed (20 g).