Just by it in Bulk.
I buy Unflavored/Unsweetened 100% Whey Isolate by Musclefeast.
They also have 100% Whey Concentrate, with No Lactose ~ that's hard to find
They are having a sale right now on the 100% Whey Concentrate ~ 5 Lbs - $35.50.
You need to get on their VIP list by signing up for their Newsletter.
That gets you a Lifetime 26% Off (VIP Discount).
Then to that you can add the Promo (100%), and that makes it about 50% off regular price.
This Sale is only good until May 13th at midnight......................................... JP
At the end of the day, you get what you pay for. There is very little money in the protein industry. The cost of raw materials are high and it is expensive to ship. Any really cheap protein supplement is going to be full of fillers. Try to save money other ways.
I am afraid there is no way for you to make your own protein powder at home - that is just too complicated and it requires special equipment, which is a bit more expensive than the protein that you could just buy. Besides, if it is a matter of finances, then you may as well opt to eat better, and use natural protein sources, such as chicken and eggs. It does not get any simpler than that. The Isolation mentioned by @muskate has one of the best, if not THE best, value on the market. It is the purest form of protein, and it tastes so damn great. It just does not get any cheaper than that.
There are tons of cheap options on protein. You can buy bulk unflavoured, flavoured like n2bm, eggs or egg whites, lean meats...bottom line is that variety is your friend here.