I have been tallying my Wii expenses since I got one in May. In many instances, I've used Gift Cards or gaming credits, but in total I have bought THIRTY-ONE games (including most of the big ones), an extra Nunchuk, a Classic Controller Pro, a Wireless Sensor Bar, a LAN Adapter, Component cables, a 2-remote charging station and the system for I believe $600 or so total OOP.
The most I have paid for a game was the $49.99 MSRP on Fire Emblem, since it was the only new copy I could find (thanks, Kmart). Aside from that, $45.99 for Metroid Other M and a $20 gaming credit is the most I've paid.
In total my expenses including all the accessories is under $400 when not accounting for the major expense in the system itself. In total I've used $70 in gaming credits at Amazon, paid for $45 worth of stuff with Amazon GCs (and could have saved another $90 out of pocket, but am saving my credit), I have used one $15 gaming coupon at Kmart and currently have $35 in coupons saved up.
The Wii can be pricey, that's for sure, but frugality, patience, and a bit of luck can net you some big savings. Anyone remember the GameStop Father's Day sale? $18.90 for a copy of New Super Mario Bros shipped after tax.
If you can avoid getting conned for accessories that are useless (the stupid Wii remote plastic tennis racket/golf club attachment bullshit) and can get some luck in finding what you want at a price you want, the Wii can stay reasonable and yet, you can enjoy a huge library of games.
In all likelihood you will want a second remote. Wii Play is $30 at PC Richard & Son ($31 shipped I believe) and has a Wii Remote with it. There are sellers on eBay that have brand new Intec Nunchucks for $6 shipped. I bought one less than a week ago and got it today- it's pretty good, especially for $6. The parts aren't Nintendo quality, but it works fine. It feels almost like a Nintendo nunchuk.
If the Wii ends up costing you hundreds upon hundreds and you are left with a small collection, you've done it wrong. I have a ton of games, and a ton of accessories, and still haven't put in what I think is a lot of cash (well, I think it is, but in relation to what I have gotten, it's not).
Reading this thread, some people understand shopping around (and apparently, following price trends) better than others.