Our current AWS setup costs us ~$500/month, last time I checked it's really expensive hosting with MongoDB Atlas, or any managed mongodb and would reach $3k+.
You should also be aware that running a two node replica-set means if one node fails and you have set a write-concern of majority (which you must do to ensure you do no lose data) then writes will fail at the client if either node is down. So you are saving money at the cost of durability. It's important you be aware of those tradeoffs.
See https://www.mongodb.com/atlas/online-archive for info on archiving and https://docs.mongodb.com/manual/core/replica-set-write-conce... for more info on how write-concerns are used in a replica-set.
The default for write-concerns on Atlas clusters is w:majority. i.e. each write must have been received by a majority of the members in the cluster before the client recieves acknowledgement.
(I work for MongoDB).