You should probably look at the nutrition information in Soylent (or similar) and compare it to your notional proposal. If your proposal is better, give it a go and do a write-up on it.

From a quick look-see, whey protein seems to carry only ~100 calories per serving, offers almost no carbohydrates or fiber and provides almost no other vitamins or minerals. So you'd need about six servings to hit a meal's worth of calories, still have to make up for the lack of carbs and fiber sources and still have to find a good multi to take with.

If a serving is $1, then we're already at around $5-6 per meal with whey protein.

With meal replacements, convenience is the killer feature. Buying whey protein, some oil, and a multivitamin is undoubtedly cheaper, but there's the non-negligible overhead of purchasing the ingredients (instead of just a Soylent subscription), and measuring ingredients for every serving.

And a supplement at best -- not a meal.