Unlike a number of other objet d' art horded, collected, fawned over and fanatically fondled, monster magazines have generally gone without much guidance as to the actual value of the tomes that are so protectively coveted. This was especially true 30 or so years ago, after the initial monster craze abated and people wanted to find out just what they had in their hairy little hands after their feeding frenzies at the newsstands and occasional fan convention.
One of the best examples of a collector's "industry" is with the monster magazine's cousin . . . comic books. OVERSTREET'S COMIC BOOK PRICE GUIDE is the hallmark and quite likely the epitome of a collector's resource for finding the market value of the items they treasure. There are other guides currently available as well, and there are at least two that I am aware of that are updated monthly.
Now, since monster 'zines could be considered more of a niche market, this might be the most logical reason for not having much in the way of price guides -- there simply is just not enough monster mag collectors as there are say, comic book or toy collectors to make it financially feasible, or even possibly rewarding enough for someone to go to the effort. That is not to say that there haven't been monster magazine price guides out there.
One in particular survived two editions in the 70s. Larry Kenton's THE MONSTERS PRICE GUIDE was a vanguard in the micro publishing field of monster 'zine guides. He used a substantial representation of dealers that sold monster 'zines to evaluate and establish a fair market price. The 2nd, 1975 - 1976 edition has an excellent Bernie Wrightson pen and ink cover depicting THE TEXAS CHAINSAW MASSACRE and a forward by Joe Kane, then editor of THE MONSTER TIMES. Here are a few pages from the second volume. Gaze at the prices and drool, fellow monster lovers!