This is true. You can get around this problem somewhat if you read reprints, which are collected in series that are available at libraries and bookstores; if you go to conventions and buy old issues from the 5-for-$1 box, or to used bookstores which also sometimes have old issues cheap; and if you restrict yourself to only actually buying new issues of a few select titles. For the most part I just read old issues; I don't even try to keep up with the new stuff anymore, it's too expensive and usually not as good. The X-Men were at their peak from about #94-175, most of which is available in reprint; likewise the best Spider-Mans and Batmans are mostly available in reprint. If you check a book about Spider-Man or Batman out of the library it will summarize the plots going all the way back to the beginning of the series, then you know what issues to look for. DK publishes a series called Ultimate Guide to different superheroes which is a really good introduction:
Spider-Man: The Ultimate Guide