Will the cost of a 2018 return for a spouse, one of a joint couple return, with no income or deductions, i.e. a "simple return" be for free ? Why or why not ?

If you use TurboTax Online and want to prepare your return for Free, you must choose the Free option when you start your return. If you choose a different option or upgrade your Free return, you will have to pay, regardless of income.

If you are preparing your returns together with a spouse then whatever you pick will apply to both of you. So again, if you pick something other than Free (or upgrade your Free return), you'll have to pay for both returns.