Those perplexed to see so many seemingly successful reality series kept off prime time schedules next fall need only see an estimate of network advertising rates to understand why.

A monthly report distributed to Nielsen subscribers, providing the cost of 30 -second commercials for more than 100 prime time series, underscores the lack of enthusiasm for the reality genre on Madison Avenue as well as the sharp drop-off in rates commanded by shows that skew heavily toward an older audience.

An illustration of the latter case would be CBS’ “Murder, She Wrote,” ranked fourth for the season among all prime time programs in terms of its average rating, but which turned up at No. 41 on a price list for 102 prime time shows airing in March.

While sources note that those published figures vary widely from month to month and are viewed with several grains of salt within the industry, they provide a reasonably accurate snapshot of how series fare in terms of their per-unit rates.

They also demonstrate why so much emphasis has been placed by the networks on the age 18-49 demographic, although ABC, NBC and even Fox Broadcasting Co. have all taken recent steps to slightly broaden their demographic appeal (in Fox’s case, from the 18-34 set to the upper range of the 18-49 grouping as well).

A show like “Matlock,” for example, averaged a 13.2 rating, 21 share for the September-through-April “official” TV season, about the same as “The Simpsons” ( 13.0/22). According to the March survey, however, 30-second spots within “The Simpsons”– which is skewed almost entirely to viewers in key demographic groups — cost roughly three times as much as an ad during “Matlock,” which attracts a disproportionately high share of viewers over 50.

Similarly, “In the Heat of the Night,” which ranked 37th in overall household ratings for the ’92-93 season, turned up 73rd based on its estimated ad rates during March.

The breakdown shows an enormous variance in prime time rates, from “Roseanne, ” which tops the list at an estimated $ 279,000 per 30-second commercial, to the Fox reality series “Sightings,” at $ 32,000 a spot.

Figures also demonstrate in part why the networks are higher on news shows (there will be 10 hours in prime time next fall, up from seven last year) than reality series. TV’s top-rated reality series, “Rescue 911,” finished 12th among all programs for the ’92-93 season with a 15.1/24 average but ranked 38th on the March survey in per-unit cost — at just about the same level as the CBS News magazine “48 Hours” (13.5/23), despite its 12% household rating advantage.

The same trend holds for other reality series, including “Top Cops” (51st in terms of average rating, 85th on the per-unit-price list), “American Detective” (47th in rating, 77th on the sales roll) and “Unsolved Mysteries” (20th in rating, in the 59th place on the March report). Those first two shows were both relegated to midseason status despite providing effective counterprogramming in their time periods.

NBC, in fact, seemed to speak for all the networks when entertainment prez Warren Littlefield told advertisers they were exercising a “reality check” in their fall lineup, with only “Unsolved Mysteries” from that genre in addition to three news programs.

The networks, currently negotiating upfront sales rates with advertisers for next season, uniformly feel they can sell comedies and dramas at higher levels than reality programs. As a result, successful shows like “How’d They Do That?””Code 3” and “Top Cops” were kept in reserve.

The four prime time services scheduled only five hours of conventional reality programming for fall, compared to 10 hours last season. There are also two new one-hour variety series, Fox’s “Townsend Television” and ABC’s “Paula Poundstone Show.”

Sitcoms soar

In addition to “Roseanne,” shows listed at garnering more than $ 200,000 per 30-second unit during March were all sitcoms: “Coach” ($ 247,000), “Murphy Brown” ($ 240,000), “Home Improvement” ($ 228,000) and “Cheers” ($ 225,000).

The newsmagazine “60 Minutes,” TV’s top-rated program overall, was next on the list at $ 199,000, followed by “Northern Exposure” ($ 194,000), easily the most expensive drama.

Again, rates should be strictly viewed as estimates based on the report, which is updated monthly.

The strong male appeal of “Monday Night Football,” which airs from September through December only, is also traditionally one of the most expensive prime time series buys. The gridcast reportedly took in more than $ 250,000 per spot last season.