Why not just use dial-up if it's a Duo? It wouldn't cost you $80, and you'd soon get tired of using something like a Duo on the web, so it's not as if your phone bills will skyrocket.
Further to your query, however, I seem to remember seeing someone, somewhere using Apple IP Gateway for a similar purpose (I think it was actually to connect a compact Mac to the web as a server), but I think it might be IP Netrouter that you want, that you really, really want, as it at least dates from the era of the internet as we know it.
The Apple IP Gateway product works, as I recall, only with MacTCP. There's a pdf manual available on the Apple site for download if you "tunnel" in via TCP/IP, if you catch my Apple-talk drift.
Further to your query, however, I seem to remember seeing someone, somewhere using Apple IP Gateway for a similar purpose (I think it was actually to connect a compact Mac to the web as a server), but I think it might be IP Netrouter that you want, that you really, really want, as it at least dates from the era of the internet as we know it.
The Apple IP Gateway product works, as I recall, only with MacTCP. There's a pdf manual available on the Apple site for download if you "tunnel" in via TCP/IP, if you catch my Apple-talk drift.