I was primarily munging data between FTP drops, S3, RDS, and Redshift, which mainly fell into free buckets for internal data transfer.
If the cost of Composer is an issue, ping me. Running a static environment _does_ have a cost, but for serious ETL it should be pretty inexpensive all things considered. You _should_ be able to use Airflow (in GCP or anywhere else) to call on other services, like S3/Redshift to operate without moving the data through Airflow, keeping network tx low.
If it's network traffic for the actual data moving to and from, that's unfortunately an artifact of how public clouds price.
Every Dilbert cartoon about naming a product is true.
Providers of that type of data essentially live in a world of "dump dataset to csv[1] periodically, place csv onto the FTP account for whomever is paying us for it currently". No deltas for changed or net new records, no per customer formatting requests, nothing. So the entire thing had to be re-processed every single time from every single vendor and then upserted into our master data.
[1] Hell, usually not even technical information was provided like the character encoding the data was stored or exported at or whether it's using database style character escapes (any potential special character is escaped with a backslash) or csv-style escapes (everything is interpreted as a literal except for a double-quote, which is escaped with a second double-quote).