RE: Have Team Members access to Field services?
You can't say that there is a cost difference between Enterprise and Team member, because you're comparing apples and oranges.
One option for you is buying Team Members Enterprise edition license. It's not either Team Member or Enterprise, it's the Team member license of the Enterprise edition. You're right that it gives you read access and "light use" to all Enterprise apps, including Field services.
Your second option is buying the Field Service app, which gives you access to all functionality of Field Service. I don't know its functionality, so I can't comment on it.
And your third option is buying a plan (Plan 1 or Plan 2), which includes the Field Service app.
I know it doesn't answer your question, but it least make clear which options you have.