When you're buying a printer, think long-term. Pay attention to issues like the page yield of the cartridges your printer uses and whether it can automatically print on both sides of a sheet of paper.
Automatic duplexing: A printer that can duplex (print on both sides of the page) automatically saves paper. That's good news for both trees and your budget. Manual duplexing--usually with on-screen prompts to walk you through turning over the paper--is better than nothing, but it's probably too much of a hassle for most people. Related: The Cheapskate's Guide to Printing
Page yield: All ink and toner cartridges have a page-yield spec that indicates how many pages the cartridge can print before it runs dry. This spec used to be all over the map, but ISO/IEC industry standards have helped make most cartridges' page yields directly comparable. That said, your mileage will vary depending on what you print and how much you print. Related: How Much Ink Is Left in That Dead Cartridge?
Starter-size cartridges: Some low-end laser and LED printers ship with "starter-size" toner cartridges that have a lower page yield than the usual sizes (and are not for sale otherwise). The starter-size cartridge will run out faster than a standard-size cartridge will, forcing you to buy a replacement sooner. Related: Which Ink Refills Can Save You the Most?