According to https://developers.google.com/appengine/docs/billing “Existing Entity Put (per entity) needs 1 Write + 4 Writes per modified indexed property value + 2 Writes per modified composite index value”
So when I insert a new element to a 1000 elements list property and put it back, it will take 4004 writes on this property. Is this correct?
Yes, thats correct. Every list property value has it’s own index entry.