Interesting. Strange, too. It cost $750 each for green card renewals when we did it. Every time we filed (the exact same) petitions, there went $750 for each person. For Aussies. Who are supposed to be some of our bestest buddies. From the initial application for fiancé visa with dependent visas, change of status from fiancé visa to temporary resident (after we got married), to conditional resident, then legal permanent resident, and finally naturalization, we filed EXACTLY the same paperwork with some slightly varying attachments on each phase, as required by HS. Each status change required a basic form, though, and the processing fee nearly doubled by the time we renewed the green cards for LPR. Don't remember the form numbers, but the only things that changed was more current photos and the dates.
Naturalization wasn't "next to nothing". It was about $850 each. Went to three ceremonies out of 4 expat Aussies. My eldest step-son hasn't naturalized yet, but pretty vehemently says he will never return to Oz to live. I just keep reminding him that Aussies have dual citizenship automatically no matter where they choose to live. Once an Aussie, always an Aussie.