Your post is much more relevant for the free agent market than the draft market imo ... In the amateur market we're mostly talking about low six figure differences on players who are very difficult to project who are forced into a market with a single bidder for the chance to be wildly underpayed if they ever make it.
It is certainly true that mistakes are cheaper and easier to absorb in the draft market, but they are also a lot easier to make. A college/high school/amateur player who is worth paying a signing bonus to or whatever may still have a very high likelihood of never being good enough to make it to the majors. Penny stocks are not necessarily any less risky or more lucrative to invest in than blue-chips just by virtue of being cheaper. In theory, the market has priced them according to their likelihood of potential returns.
If there is some particualar reason to believe, for instance, that the draft market is undervalued or that the risks of signing young prospects are exaggerated in the marketplace, then it may be true that the price premium for the certainty of established players is as a whole inflated compared with the true level of talent and probable performance at the draft/amateur level. In that case we could say generally that the market for baseball players has not priced risk accurately and those imbalances could be exploited by a team that spotted them.
However, without having any particular reason to think that the market is undervaluing the amateur/draft market as a whole, there is no reason to think that players who are cheaper from a dollar amount point of view are necessarily proportionately less risky. And risk is the name of the game when it comes to player evaluation at any level. We value them according to their probable and potential contribution towards the big-league club, primarily. Players who have proven their ability to win in the big leagues are extremely valuable, and players who have a small chance of contributing are worth very much less, or nothing.
So, as a grossly oversimplified thought experiment, let's hypothesize that for instance the Red Sox were able to secure a generic class of "better" draftees by offering $1m signing bonuses to 20 players who otherwise would have signed earlier. Ignoring luxury tax, roster spots, etc: would the difference between those 20 draftees and the 20 the Red Sox *would have* signed without bonuses ultimately contribute more value to the club than signing a single established player to a $20m contract?
If the generic answer to that question is yes, then it would be correct to say that the draft is a better place to spend money than the FA market. If the answer is no, then the draft is actually a riskier place to spend money, even though the increments seem trivial. If the answer to that question is that either move would generate about the same on-field production, then the market has priced risk appropriately (there are actually a lot of peripheral considerations such as present vs future costs and so on but whatever).
I don't really know whether the draft/amateur markets are undervalued (they may be), but the simple fact that the players are cheaper per-unit does not in and of itself make them less risky, counter-intuitive as that may sound.
I would argue that it is, in fact, impossible to be "too rich to get poor" in a rigged monopolistic market.
...Uh, I think I agree (?)