My pruned node finished syncing in about 10 hours (with HDD, Xeon and 16 GB RAM). It's now uploading around 15 MB per hour, so it looks like it's going to be around 10 GB per month.
A quick check after a restart shows it's downloading more than it's uploading:
"totalbytesrecv": 474706,
"totalbytessent": 344889,
If it stays that way, it doesn't provide a net contribution to bandwidth.
Update: it's uploading more than it's downloading:
"totalbytesrecv": 2090982,
"totalbytessent": 5183814,
That means I'm contributing bandwidth