The problem is the demand; for all the work put into designing the ISA bridge and making it cost-effective to produce, how many of these do you think have sold, versus the original silicon that could 1. integrate the bus logic and 2. ship in quantities great enough that I own more generic Realtek cards than computers?
(In other words, the expense isn't the ethernet controller; if it were just an ISA bridge, a-la GoldenGate, it would still be just as expensive and do less.)
I do think it's a shame that a couple bucks couldn't have been knocked off the original 5? years ago; lack of connectivity has literally kept the Amiga isolated. (Nobody had 'obvious' Plug-'n'-PPP solutions then, and the slightly larger crowd of holdouts left weren't ready to tether their machines to PCs.)
