Platinum games (and counterparts) historically had to pass a certain number of sales within 1 year. The number of sales has been anywhere from 250k-1m units. Typically a game won't go platinum until at least 1 year on the market.

However, just because a manufacturer offers a platinum range of games does not mean that all games that meet the criteria will be platinum right away. Why waste profit on a game when it is still selling well? If everyone knows the price will drop within 1 month, why buy now when you can save $10 later on a brand new game?

In your case, I'd look at some of the best overlooked games that can be found really cheap, like Super Paper Mario and Wario Land: Shake It!

if you go here, this is a great site for finding used games cheap. You can sort by price by clicking the column header and there are tons of good games for $25 or less.
http://www.videogamepricecharts.com/console/wii

EDIT: some more great games for $25 or less

Metroid Prime 3
Mario Super Sluggers
de Blog
New Play Control Pikmin
Klonoa
Excite Truck
Big Brain Academy: Wii Degree
Rayman Raving Rabbids (1 & 2)
Mario Strikers Charged
The House of the Dead 2 & 3 Return
Okami
The Godfather: Blackhand Edition
Truama Center: New Blood
Truama Center: Second Opinion
Zack & Wiki
Lego Star Wars the Complete Saga
Pinball Hall of Fame: The Williams Collection
Resident Evil 4: Wii Edition
No More Heroes