I took a look at the player universe - with 24 teams the only way you are going to blow past $120 million is if you go overboard on at bats or innings.
As an example, you can get nine batters and five starters averaging $7 million each, seven RPs averaging $2.5 million each and four bench players at $1 million each and still be under cap. There aren't 24 teams worth of those players to be drafted.
Maybe 2004 Bonds creates a problem for someone, but otherwise $120 million should be ample room.