Yeah, unless you're willing to try the emergency turnaround spots (if there's a cop speed-trapping, you'll get a hefty ticket, and by the time you've slowed down enough to use the turnaround, it's probably too late), you'll probably end up more than paying the difference in tolls.

If you already passed through the Lee Toll Gantry by MM 10, you'll have to drive all the way to exit 3 to turn around, go to the cheaper McD's, then pass through the toll readers around MM 10 to turn back around at exit 10, then incur another toll after getting back on the highway.

You're better off exiting the highway to eat at the McD's off exit 10, which is still $0.40 more expensive than the eastbound service plaza location, but still $1.00 cheaper than the westbound service plaza location.

Edit: Turns out I was wrong. I calculated the difference, and paying the toll thrice would cost $0.75-$1.80 (vs. paying $0.25-$0.60 once), depending on if you have E-Z Pass MA, E-Z Pass (other states), or pay by mail.

Depending on how you pay the toll, you'll save $0.20-$0.70 paying thrice to visit the eastbound McD's vs. paying once to visit westbound McD's, but the cost of gas might make it a wash. If you're paying by mail, the best choice is exiting the highway, paying $7.09, then re-entering. (That said, if you're going eastbound, stopping at the service plaza wins every time.)