So, here's something that just occurred to me:
It seems that... since you are required to buy hits for things done in downtime during Missions... it pretty much behooves you to have your total DP for Binding at
exactly a multiple of 4 after specialties and Foci are accounted for. Any fractions are, essentially, wasted since you have no way of making use of them.
It also means that you should Summon/Bind Spirits during down-time at a Force equal to 4X-1 (i.e. 3, 7, 11, 15, etc) since
they have to buy their hits, too, and those get you your best Force-to-Resistance-Hits ratios.
Thus, a Magic 6 + Summining/Binding 6 Mage with a F4 Power Focus, and a matching Specialization and Mentor (+4 total) has a DP of 20 and buys 5 hits for his chosen specialty, resulting in a nice, beefy 4 services when Summoning or Rebinding a Force 7 spirit and 3 on the first binding, for 7 services out of the gate. If you've got more than 8 in your Drain Resistance pool (which should be a no-brainer), you auto-soak the DV for it, too, since 1 Net Hit = 2 DV.
Pump that up to 11 (no pun intended) and you still get 3 & 2 for Services. Get your DR pool to 12 and you only eat 1P for the summoning and 1P for the Binding, a light enough price for 5 services for a mammoth of a Spirit.
If you assume the Mage has some Initiation and Summoning & Binding pools of at least 21 to start, they can even get 2 & 1 from a Force 15 spirit, though they'll have to eat 3P per step to make it happen unless they're doing something hinky with Qualities or Cyberware. At this point, though, I think the cost:performance ratio has start slipping very badly thanks to the binding expenses.
Thoughts?
EDIT: Fixed a math error. 2*2 = 4, not 3. You eat some damage with the F11 spirits.