At that age, there isn't really a guide. A sled that old is worth what someone will pay. With the huge difference between sleds that have been properly maintained or even restored to the same model sitting under the straw bales in Grandpa's garage, no one can accurately identify a true value.
Here's a web site that a guy put together that shows a lot of vintage sleds that have been sold, with their actual selling price. The site is not very user friendly, and you may not find your sled (I didn't find a "search" feature), but the prices are actual prices someone paid.
1_A_SLEDS_WORTH_TEXT_PAGE
Good luck. Hope you find one like yours in there!
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2007 RMK 700 155", 2008 RMK 600 155", 2006 RMK 600 144", 98 Indy 440, 95 Prowler 550 2-up, 1990 Indy 650
RIP, destroyed by fire 1-22-2008

04 RMK 800, 05 RMK 600, 02 RMK 800, 97 RMK 700
http://www.avalanche.org/accidents.php