Cost is less but it's less flexibility and more time on the train. It's really up to the individual, where they are coming from to reach the station and how much their time is worth. The cost difference in a monthly pass works out to about $3 per working day, plus $3 cheaper parking per day or about a total of $120/month.
Looking at the morning service, Mansfield's slowest trip in is 46 minutes to South Station with express train 808 clocking in at only 31 minutes. There are 9 trains before 10am, all stopping at Back Bay and 4 at Ruggles. By comparison, the fastest ride in from Foxboro is 54 minutes. Of the 3 trains before 10am, 2 stop at Back Bay and only 1 at Ruggles.
808 Mansfield 7:45 to South Station 8:16 (31 minutes)
740 Foxboro 7:30 to South Station 8:24 (54 minutes)
The evening is similar between 3-7pm, with Mansfield having a max trip of 43 minutes and a few clocking in at 35. There are 7 trips and all stop at Back Bay and Ruggles. Foxboro's fastest is 55 minutes, with 3 trains and only 2 stop at Back Bay and Ruggles.
823 South Station 4:53 to Mansfield 5:32 (39 minutes)
745 South Station 5:03 to Foxboro 6:05 (62 minutes)
Assuming equal drive time to either Mansfield or Foxboro for a commuter making the competing trips above, the cost savings of $6/day is at the expense of 46 minutes of your time. To put it another way, you made $6 in 46 minutes. The equivalent of getting paid $8/hr.
Again, it's all up to the individual and there will certainly be people who make the shift. But if I currently afford the cost of riding from Mansfield, I'm likely not changing my commuting pattern unless I live considerably closer to Foxboro.