There’s no /minimum/ monthly fee. There are charges for transfer in and out as well as for storage. In the scenario you describe, you’d pay for the upload, the download, and for storage as long as the data is in Tarsnap.
Transfer is billed by the byte (currently $0.00000000025/byte) and storage by the byte-month (currently $0.00000000025/byte-month).
So for your example, you’d pay $0.25 for the upload, $0.25 for the download, and $0.25 for each month the data is stored (supposing a GB is 1,000,000,000 bytes).