We are looking to replace our current CRM with Microsoft Dynamics 365 and are seeking help in calculating our storage costs based on the number of records we intend to migrate to Dynamics365. We are looking at DataVerse but wonder if there are lower costs alternatives? Would any peers (ideally in the non-profit community) on Dynamics 365 be able to share estimates that would help us to better estimate our storage costs?

first of all you need to understand and calculate your consumption and your current data size which can help to calculate the size

Part 1 of the question on CRM Migration
1 Assumption while advising you on the solution 
Current CRM is on Premise CRM

I would advise you to follow below links which I followed in my migration and almost all the pointers were useful. 

https://learn.microsoft.com/en-us/dynamics365/fasttrack/migration

Part 2 of the Lower Cost 

This is purely a use case base but I have done was migrated limited data on data verse and reporting data in csv file into the data lake