James,
Good to hear from you. I did a search on www.newegg.com for ethernet hubs and found some, like you said from $20 to over $200. Like all things, when the supply is short the cost is high.
Even though, I understand your desire for 2 separate IP networks. Your solution on using hubs is not a good one for the sake of what you want "absolute security." By using a hub, any one single port still gets the any electrical signal sent from any one port. This will be true regardless if you put all of your devices on different IP networks because of the basic functionality of the hub.
You've got a few options though like you've mentioned. The only way to guarantee that your two networks will never be able to directly connect to each other directly is to put them on completely separate equipment. Or you can use two hubs, each directly connected to your router on separate router interfaces.
I would recommend the usage of a switch with vlans. The cost is a little more but as you're seeing the cost of hubs are not a lot less anymore. With the switch, you can create your 2 separate vlans and use a single router port (100 Mbps or higher) along with access control lists on your router to keep the traffic separated.
Cordially,
Ronnie Wong
Host, ITProTV