Old games that are in excellent condition can potentially be worth a lot of money, so it is not surprising that you have found Archon I sell for a lot of money on eBay. If you are adamant about obtaining this game, you need to research how much the game has been selling for over the last year or two and be prepared to pay approximately the same amount for the game. It will require patience and self restraint, but you should eventually be able to purchase that game you so desire.
Racketboy.com is my favorite community driven retro games website out there. They have some great guides to help you get informed on the best way to proceed with retro game purchases:
They also have a thriving forum with several members that are avid retro gamers and game collectors (some of them even own video game stores), and many of them have channels that they can call upon to find rare games for a fair price.