The most desirable VCS model is the original heavy sixer, which tends to sell for a bit more than other versions of the hardware.
In addition, Atari's own 7800 console -- besides utilizing its own software and accessories -- is nearly 100% compatible with VCS software and most add-ons, making it a great choice for collectors who want to play the games, particularly since it also has a platform-specific multi-cart that can be used for running backward-compatible software.
Besides all the variations Atari itself produced, there have been many clones and add-ons for other systems, most commonly from Sears and Coleco.
As was their corporate sales policy at the time, Sears rebranded Atari hardware from the original heavy sixer right through a sleek custom unit with a unique case and combination joystick and paddle controllers. Sears sold them under the Video Arcade name, also rebranding games: Indy 500 became Race, for instance.
Steeplechase (1980), a paddle controller-based horse racing game, Stellar Track (1980), a shooting game set in space, and Submarine Commander (1982), an undersea target game were the only cartridges released with the Sears branding that didn't also have an Atari counterpart.
Coleco released both the stand-alone Gemini console, which also featured a single joystick and paddle combination -- one above the other -- and its popular Expansion Module #1 for the ColecoVision console and Adam computer. Each allowed for high VCS game compatibility. Resulting lawsuits forced Coleco to pay royalties to Atari for every system and expansion module sold.
An unboxed, working system with several loose cartridges can usually be had for well under $50, with expansion modules for other systems usually selling for far less. You can find loose games -- especially the more common ones such as Boxing (Activision, 1980) and the Stern arcade conversion Berzerk (Atari, 1982) -- for about $1.
On the other hand, rarer games can cost into the tens of dollars or even hundreds, with titles such as the first voice-enabled VCS game, Quadrun (Atari, 1983), the double-sided cartridge Tomarc the Barbarian/Motocross Racer (Xonox, 1983), and the jumping and matching game Q*Bert's Qubes (Parker Brothers, 1984), selling for far more than their more plentiful counterparts.
Since many of the same titles received different text and graphical label variations over the years, this can factor into a game's price, as there are collectors who like to get all variations. Besides color variations (especially Atari's with silver and red), most box types were generally the same, save for the earliest Atari and Sears cartridges, which came in gate-fold boxes for a short time and are subsequently worth a bit more.
Like nearly every system, there are prototype games that were either finished and not released or only partially completed. However, because of the popularity of the system and the timing of The Great Videogame Crash, the 2600 has a particularly large collection of unreleased games, with new prototypes occasionally discovered to this day. Once recovered, the homebrew community often makes these lost titles available in some manner, even on cartridge.