Getting good bullpen results from starters on the farm is just as likely, if not more likely than signing mid to low tier free agents. Unless you're a top end bullpen arm, you're never more than 10 innings from being at risk of a DFA. They are not reliable by nature and we've seen a thousand times that changing, usage, teams, cities, managers, teammates or ballparks makes differences you can't even begin to predict for them.
Even if the Twins spent 20M on the pen this year, it wouldn't have been a good pen. This is the one spot on the team you have to build through familiarity and patience.