When deciding to buy a road sign from eBay, you typically want to find and get the best deal possible. However for this thread, the goal is to find the most expensive road signs.
To start things off:
https://www.ebay.com/itm/Genuine-H-1-Hawaii-Oahu-Interstate-Sign-Highway/283545679021?hash=item4204a3b4ad:g:7jkAAOSwODBdJ966
UPDATE: This sign has been sold.
There was a guy trying to peddle a CA 1 BGS for 55k a short while ago.
I'm pretty sure I saw that one. I get it's a BGS but 55k!? You could buy a new car for that, or even a house!
Here's some more high priced interstate shields:
https://www.ebay.com/itm/VINTAGE-SOUTH-CAROLINA-I85-INTERSTATE-85-HIGHWAY-ROAD-SHIELD-SIGN-TRANSPORTATION/273930336381?hash=item3fc785387d:g:V~kAAOSw4aRdLMD1
https://www.ebay.com/itm/Washington-interstate-5-highway-marker-route-sign-Seattle-Olympia-Tacoma-1965/352728700083?hash=item522044d4b3:g:G3QAAOSwgW5dNgcE
UPDATE: This sign has been sold.
I suspect having the State name on those shields is a price driver. Personally I would be game in the $50-$80 range but you'll never see that on eBay.
I paid $80 for my I-94 shield, which came from eBay. I jumped at it as soon as I saw it because you never find Illinois state named shields on eBay. And you're right, it's very rare you see any shield, let alone one with the state name, for under $100.
Here's another one.
https://www.ebay.com/itm/I-95-interstate-SIGN-Pennsylvania-24-x24-Genuine-Used-aluminum-highway-route/143337075885?hash=item215f8e34ad:g:khgAAOSw8EhdN8ni
I get it's next to impossible to find these, but I think it's still expensive.
https://www.ebay.com/itm/I-80-S-Ohio-Interstate-SIGN-18-x-18-highway-Route-Shield-80/223601545839?hash=item340fb0d26f:g:E5gAAOSws8NdO2D8
UPDATE: This sign has been sold.
Here's another high priced state named shield.
https://www.ebay.com/itm/Genuine-Vermont-Interstate-89-Sign-Highway-Road-Street-36-x-36/333279798348?hash=item4d9906244c:g:NaUAAOSwXoZdPdIP