What are our pricing options for OMS to for Windows 10 upgrade analytics? (We have SA already)

Agree with Marc, and adding some nuance: As far as I am aware there is only one (uncommon - requires workspace customization) scenario where you could incur OMS charges related to Windows Analytics records in your workspace. Here is the situation as I understand it today:
1) Ingestion costs: The "IsBillable=false" records such as Windows Analytics records don’t count towards ingestion costs (e.g. per node plans or per GB plans) and don’t count towards ingestion limits (e.g. free plan)
2) Storage costs: For the default retention period of a given plan (e.g., 1 month for Standalone plan https://azure.microsoft.com/en-us/pricing/details/log-analytics/ ), the "IsBillable=false" records such as Windows Analytics data do not incur storage costs, *but* if you customize you workspace to retain data beyond the plan default, then all records (including Windows Analytics) retained beyond the plan default should be charged at the normal extended storage retention rate (e.g. $0.10/GB/month as of today). Note that in some cases this extended retention billing may not be enforced today, but should be expected to be enforced in the future.

tl;dr : Windows Analytics data is never expected to incur OMS/Log Analytics costs, except in the uncommon case of customized data retention beyond plan defaults.