“You get what you pay for” is a maxim that is just as true in hockey as it is in real life.
With regards to prospects, the best are rarely free. High draft picks have high value on the trade market, of course. The draft picks 217 players, and then only around 20% make the NHL on at least a semi regular basis. Even the ones teams pay for aren’t always that good, but imagine what that says about those that don’t get drafted.
Good prospects do go undrafted, that much is true, but given the larger general pool of undrafted players who never made it versus the ones who did, undrafted stars are in an extremely tiny minority.