You have a couple of realistic options here:
[ol]
[li]Just wire it. This is by far the best option because a) you won’t have to enter your network key with a controller, b) it won’t ever cause issues during online play, and c) it’ll work far better when you start streaming multimedia from your computer to the TV via 360. (Plus, if you live in a house with a basement, you can easily pull the wire.) This, BTW, is what I did.[/li][li]Buy the unit. Seriously, you’re going to have less problems with this then you will others. It’s designed to work with the 360, and it’s about as painless as 360 wireless can be.[/li][li]Buy a wireless network bridge. This used to be the solution before the 360. It essentially connects to your wireless network and gives you a jack you can connect to your console or a hub or switch. It’ll likely work, but you’ll spend more time configuring it than you will the 360 wireless, and you may need to tweak settings to get it right. I had one for a printer a number of years ago, and it was finnicky. That said, they’ve probably gotten better (and cheaper) over the years.[/li][/ol]
Hacking together some kind of temporary solution isn’t going to cut it. You fire up your console to relax, not to play “network support.”
FWIW, the no-built-in-wireless isn’t an insult. Mine’s wired, so why would I want to pay extra to have wireless capability that I don’t need? And $100 isn’t really injury for this. It’s a specialized piece of equipment that makes life easier.
BTW, it’s available almost everywhere for far under $100. Amazon lists it for $77 and you can find 'em used at GameStop and the like cheap, too.