Wow. That's outrageous. Here's one on eBay right now for $20 "Buy It Now"
http://cgi.ebay.com/KEYSPAN-USB-Twin-Se ... dZViewItem
There are several others that start at .99 and the completed auctions I looked at were not over $45 max. The DE-9 serial adapters, which may also work with the correct cables are plentiful and have not sold for over about $25 highest auction price.
Nevertheless, you have other options for networking with a Plus. Technically the http://www.mac128.com/terminal
tutorial is a file-sharing method via serial terminal transfer. specifically tailored to the 128K & 512K for which options are severely limited. The Plus is capable of being networked via AppleShare, but it is a slightly more involved method when connecting to anything running above OS 9. See this page: http://www.vintagemacworld.com/netmain.html
as it has links to just about every Vintage networking page there is as well as giving some very good tutorial tips itself.
That said, even traditional AppleTalk networking requires an investment in hardware. LocalTalk connectors, cabling, AppleTalk to Ethernet bridges depending on your network. In particular I was using my Mac 512K with a LocalTalk connector, cabled to an Ethernet bridge and then CAT-5 into my G3 running Mac OS 9. The catch there is, the G3 couldn't see the 512K – it was only one way from the 512K. Moreover, you can only access a small volume. I've forgotten exactly the limitations. But the whole thing would crash if I tried to access more than a 5GB drive.
So, the MacTerminal method is compatible up to Leopard with bi-directional transfer and no volume limitations because you aren't actually mounting any volumes. It is definitely slower than AppleTalk, but when you are talking about 400K disks it's not so bad (which is where the tutorial was aimed). And it may be slightly more expensive than AppleTalk. But certainly not $125 worth. Unfortunately, the only way to get a serial connection into a G4 is with some kind of USB converter or AppleTalk over Ethernet. So sadly, you have to invest in more hardware no matter what you do.
Since a Plus can run up to System 7.5.5, you can easily run TCP/IP stacks per Phil Beesley's site and forego AppleTalk or serial transfer methods altogether. Instead pick up an EN/SC adapter and run Ethernet right out of your SCSI port which is also faster than AppleTalk. It still takes some effort to connect to OS X, but it certainly reduces your hardware investment and simplifies the entire process. Plus it's a true network and 7.5.5 bypasses the earlier system's volume limitations.