Here's my guess for sales numbers by year:
In 2001, the Dolphin is launched only in Japan and America, in late November. Due to a combination of high price and supply constraints, only about half a million units are sold during that holiday season. Nintendo gets really worried by these low sales too; hence, a $50 price cut in May.
Fortunately, the Dolphin picks itself up in 2002, selling better as Super Smash Bros Melee spreads through word of mouth and people get Dolphins to play Super Mario Sunshine and Metroid Prime. However, only about 3 million units are sold that year, as the price is still too high for most.
The turning point is in 2003, ironically around the time the GameCube began to falter in OTL. A new Zelda game is a big deal, and is a system seller for many people, but the game-changer is the eShop. It, combined with another price drop in the first half of 2003, makes the Dolphin seem like a surprisingly good deal. Think about how people buy $500 iPads to play $1 games. In this scenario, there are actually two killer apps: Animal Crossing, which at a mere 17 MB could have been sold digitally, and Pokemon Battles, essentially a tech demo for the Game Boy Player that let people battle their Pokemon Ruby and Sapphire games online. These two games are much bigger deals than eShop or WiiWare games in OTL. Meanwhile, the release of Mario Kart: Double Dash at the end of the year essentially completes the Dolphin's library, and gives more casual gamers an excuse to buy Dolphins. Overall, a bit under 4 million units are sold in 2003.
2004 is the first year that the Dolphin outsells the GameCube, for a number of reasons. First, the Dolphin finally goes down to a mass-market price, $250. Second, games like Melee and Double Dash, which were already released, reach critical mass. Third, the Dolphin's at least mediocre following in Japan, combined with the ease of porting over PS2 games, makes it the home to games like Dragon Quest 8 and Metal Gear Solid 3. Nintendo's mended relationship with Square Enix and always positive one with Kojima help that. Fourth, a lot of good exclusive games, from Paper Mario to Pikmin 2 to Metroid Prime 2, are released.
2005 was the year when the GameCube fell off a cliff. The Dolphin, however, continues to sell well. Indeed, about 5 million units are sold a year from 2004 to 2006. In 2005, sales are kept up by new models, cheaper prices, and better 3rd party support than in OTL. Resident Evil 4 is something of a killer app to many hardcore gamers, especially since the eventual PS2 port is of inferior quality. Around the end of the year, the eShop also picked up more steam, as services like Xbox Live Arcade allow for developers to make downloadable games for more platforms.
2006 is the beginning of the end for the Dolphin, but at least it wasn't the abrupt end the GameCube had. Much like how the PS2 kept on selling to a wider audience after the PS3 and 360 came out, the increasingly inexpensive Dolphin kept on finding new ways to sell old standbys like Super Smash Bros Melee and Mario Kart: Double Dash to new audiences. New colors, bundles, and models are pumped out. Meanwhile, the continuous success of the PS2 combined with the Dolphin's survival gives developers, and thus consumers, a viable alternative to entering the HD age.
2007 is the Dolphin's last great year. The PS2's replacement with the PS3 cuts back on the number of multiplatform games it receives, and the Dolphin starts showing its age compared to games like Bioshock and Halo 3. Still, a few major games are still released for it. Examples include Super Paper Mario, which was originally intended for the GameCube in OTL, Metroid Prime 3, thus making the Prime series fit on a single platform, and, surprisingly, Guitar Hero 3, which rocked virtually every console of 2007. 4 million consoles might be sold this year.
The Dolphin continues to be in production up through early 2010. In 2009, the price is cut to $100, making it a budget system. Sports, Lego, Licensed, and various multiplatform games are released for it. Sales are cut in half or so a couple of years in a row.
In total, the Dolphin sells around 30 million units, of which half were sold from 2004 to 2006.
