Even though I know nobody around here other than me likes hoops, the NBA system isn't too bad. The Knicks pay an astronomical amount out in salaries, but they pay a dollar-for-dollar luxury tax on everything above ~ $60M (which means they pay about $50M in tax). There's both a regular cap and a luxury tax threshole, which is essentially a hard-cap; very few teams go over that level. The regular cap allows players to re-sign with their existing teams regardless of whether the team is under the cap or not. There is a fair amount of player movement, but not as bad as in years past and no more than in baseball. And, most teams tend to spend around $45M - $60M.
The NFL system works well, too, but it requires a shift towards signing bonuses and away from guaranteed contracts. Seems more likely that the NBA model would be realistic for MLB (well, not really, because the union is far too strong).