Since you live in America this should be easy, I'll be going by listings based in the U.S.
All three games were made available on the Wii U éshop for $10 each.
If you don't have a Wii U or want to play them physically on a DS/3DS, buying the original versions costs more or less depending on where you get it from obviously. Since these games are old DS titles they're not sold new anymore so any cheap listing you find somewhere like GameStop or Amazon is selling it used with their own price in mind.
All three games have similar ratings from places like IGN and Metacritic (they all orbit around 7/10), but the highest rated one seems to be the aptly titled Pokémon Ranger based on those ratings alone. That said, I'm pretty sure the first game in the series is considered the worst by the fans who've played them, and Almia is the fan favorite.
The only available one I can see on Amazon is Shadows of Almia for $25, but Ranger and Guardian Signs are listed as $25 on GameStop's website, with Almia being $30.
So each of them can be bought for $25. Conveniently consistent. If you want to find them cheaper than that you'll have to do your own online/local shopping. Sometimes GameStop's website lists them as more than they'll be in your local store.
