I think I mentioned this the other day in another thread. A wireless bridge is the easiest way, but decent ones cost about 80% the value of the official wifi adaptor. It is a crappy solution anyway as you still need a mains outlet to plug the device into (obviously the 360 adaptor is USB host powered).
Some people suggest buying a cheap router/access point and using that as a bridge. Still a fiddly solution like an ethernet/wireless bridge as that is one more device on your LAN you have to configure tbh.
Any solution apart from the official one requires you to configure an extra device on your LAN (i.e. through web browser etc) and requires a power point (and possibly device to be left on all the time or turned on specially when you want to use 360 online).
There is no cheap/easy way to use wifi unless you are willing to leave other devices on. The MS wifi adaptor is an over-expensive add-on (I think RRP has stayed the same for 3 years!!!), people do not like to admit this, so they suggest fiddly work around solutions.
If you don't mind the hassle do it with a bridge or a router, but for the amount of money you would save and added to the fact you have to have another device plugged in, I would just but the MS one tbh. Don't think you can put a price on simplicity and ease of use (well MS think it 59.99

)
I see you can get these for 52.99 delivered online.
rp2000