For the first 72 hours you can build your dream metropolis for just $4.99 before eventually going up to its regular tax rate of $19.99. Don't forget the Network Addon Mod.
Any true Mayor knows there's just far too many awesome community developed mods and models to ignore for SimCity 4, so you'd best check out Simtropilis.
"Lauded as the crowning achievement of the genre, the game allows for an in-depth look into the challenges of urban morphology with extraordinary attention to detail (including real-life city landmarks and building models inspired by 20th century architectural styles) and features like traffic management, or public funding distribution."
SimCity 2000 Special Edition is already available on GOG.com, but maybe you want to upgrade?