Don't "eat it", drink it! Don't limit yourself to protein-only shakes or the over-priced, over-processed mass gainers. Here's an inexpensive, nutritious and calorie dense "meal replacement" shake recipe I use 2x/day (currently averaging 3,800 calories/day):
1. Soy protein isolate from Honeyville ($.26/serving of 35 grams)
2. Milled flax seed from Honeyville ($.14/serving for 14 grams)
3. 1 cup of dry oats ($.18/serving)
4. 1/8 cup of Trader Joe's flake coconut ($0.15/serving)
5. 2 TBSP Peanut Butter ($.12/serving)
6. 1 banana ($.19/banana at Trader Joe's)
7. ~16oz water (essentially free)
Totals = $0.94/serving, 965 calories, 61 grams protein (25%), 40.5 grams fat (38%), 91 grams carbohydrates (38%). 1/2 cup of oats should be all that you need for each shake, depending on your activity level. I buy the protein, flax and oats in bulk. Could probably make it even cheaper if I got the peanut butter and coconut in bulk as well.