Though aimed at adults, Dick Tracy's cartoon-like quality made the movie appealing to kids. Nonetheless, parents who plan to stuff stockings with Tracy, released on video Dec. 19, may be shocked when they see its $92.95 price tag when other kiddie favorites, such as Teenage Mutant Ninja Turtles and The Little Mermaid, go for ''sell-through'' prices under $30. Why do some major movies cost so much and others so little? Video companies set prices according to research that purports to show which videos viewers will rent and which they'll choose to collect. Movies with strong ''intent-to-rent'' ratings are shipped in smaller numbers and those with high ''intent-to-buy'' scores are released on the cheap in large volume. Kids' tapes are the most buyable, says Video Store magazine's Bart Story. Then there are three exceptions: Rocky Horror Picture Showdevotees surprised CBS/Fox by gobbling up nearly one-third of the copies shipped out Nov.8, despite the $89.95 price. Dick Tracy showed good ''buy'' scores, but a Disney spokesperson says the company decided not to price it low, as there were already too many sell-through titles out at holiday time. But, if fans still want to own Tracy, Disney will send a rebate and a new box to any who buy a marked-down copy at stores.