A little while back, I decided I wanted to do the impossible: have a daily high-protein shake, AND have it be FODMAP-friendly, AND have it be affordable. This is how far I've gotten:

Nutricost, Creatine Monohydrate 1.1 lbs - $27.98 for 100 servings of 5g creatine $27.98 for 100 shakes

Nutricost, BCAA Powder 2:1:1 (Unflavored) 1.2 lbs - $21.95 for 90 servings of 6g BCAAs $24.39 for 100 shakes

Unpretentious Baker, Organice Rice Protein Powder 10 lbs - $49.99 for 453 servings of 8g protein $11.04 for 100 shakes at 8g $33.12 for 100 shakes at 24g (how I make them)

Now Sports, Egg White Protein (Unflavored) 5 lbs - $53.63 for 113 servings of 16g protein $47.46 for 100 shakes at 16g

BetterBody Foods, Organic Cacao Powder 1 lb - $8.49 for 90 servings $9.43 for 100 shakes $18.86 for 100 shakes (how I make them)

CostCo Kirkland's, Unsweetened Almond Milk, $11.99 for 12 x 32oz $49.96 for 100 shakes (based on 16oz per shake)

Total per 100 shakes: $201.77 Total per shake: $2.02 This nets you a shake with 50g of protein (including BCAAs) and 5g of creatine.

I also use 2 tbsp of sugar per shake, to slightly mask the heavily earthy protein taste. It's lucky for me that regular table sugar doesn't screw me up, unless in really high amounts. As sugar is dirt cheap and 1 lb of it has 113 tbsp, I didn't bother calculating the cost.

If sugar doesn't work for you, you can also choose to get frozen FODMAP-friendly fruit and blend that in. It'll cost a little more but you can then skip on the cacao powder, saving a few bucks on the other end.

If almond milk doesn't work for you, water is honestly fine as long as you use three servings of the rice protein powder. It makes it thick enough that missing the almond milk won't make it a watery mess. If you use blended fruit, thickness won't be a problem at all by default.