@cloudspider I am running Kadira on digitalocean now with a two member replica set: one $20 droplet and one $5 droplet. I can give you my numbers.
For 5 days of usage, the database sotrage requirement balloons to 700-850mb for 5 days, so it will be about 5gb for one month storage for a free user. Hence the cost for each free user will be about $2 just for storage.
Please keep in mind that I am hosting the mongodb on a droplet, which means no security except port protection, no backup, no alerts. I am doing this because I don't see the kadiraData db as some sensitive secrets. Of course, you can roll these on your own after some time.
For the longest time I don't know why I need to look at the backup data, I think the real time data is the most useful thing. Perhaps you can consider to give out the free tier with one day backup like arunoda did.