30M in IFA should net you, on average, two ML-quality players per year. If you chart that out:
2 in their "not counting) year: 0.5 M total
6 in the 1st, 2nd, or 3rd pre-arb years: 2.5M total
2 in their first arb year: 4M total
2 in their second arb year: 11M total
8 currently under long-term contracts (average of 7M per): 56M total
So that's about 74M for 20 players, which is essentially your 9 starting bats + 2 bench bats and 5 starters + 4 primary relievers. The other 3 bench players and 2 relievers shouldn't cost much more than 5M to put together.
So it's a pretty sustainable system, although it does leave you vulnerable if your world goes totally IFA crazy. I just think building through IFA is so much more predictable than the draft. Top draft picks are definitely more efficient (when you think that a top draft pick will cost you ~20M in bonus + scouting whereas a top IFA will cost 40M+ in bonus + scouting), but if you are a consistent playoff team, the only way you have access to All-Star talent is through FA and IFA.