Google appengine, least expensive way to run heavy datastore write cron job?

I have a Google appengine application, written in Go, that has a cron process which runs once a day at 3am. This process looks at all of the changes that have happened to my data during the day and stores some meta data about what happened. My users can run reports on this meta data to see trends that have happened over several months. The process does around 10-20 million datastore writes every night. It all works just fine, but since I have started running it I have noticed a significant increase in my monthly bill from Google (from around $50/month to around $400/month).

I have just setup a very basic taskqueue that this runs in, I have not changed the default settings at all. Is there a better way that I could be running this process at night that could save me money? I have never messed around with the backends (which are now depreciated) or the modules api, and I know they've changed a lot of this stuff recently so I'm not sure where to start looking. Any advice would be greatly appreciated.

Look at your instances at 3am. It might be that GAE spins up a lot of them to handle the job. You could configure your job to make it run less paralel so it will take longer but perhaps it will need only 1 instance then.
However, if your database writes are indeed the biggest factor this won't make a big impact.

You can try looking at your data models and indexes. Remember that each indexed field costs 2 writes extra, so see if you can remove indexes from some fields if you don't need them.

One improvement that you can do is to batch your write operations, you can use memcache for this (pay the dedicated one since it's more reliable). Write the updates to memcache, once it's about 900K, flush it to datastore. This will reduces the number of write to datastore A LOT, especially if your metadata's size is small.