On Spider-Man, the cheapest way to catch up on him is to read reprints of the originals. These used to be published monthly in a series called Marvel Tales, which you can find back issues of if you go to a comic store or if you search online--this site has most of them indexed for decent prices:
Some reprints are also collected in this series, which is available at bookstores or used from Amazon starting at $5:
The Amazing Spider-Man (The Essential Spider-Man, Volume 1)
This series reprints the stories in black-and-white, which is why they're so cheap. Marvel Tales reprints are in color. There have also been mini-sized paperbacks reprinting the first 20 or so issues very cheaply in color but in smaller size; these are hard to find but a comic store might have them. There are also more expensive reprints, but that's the cheapest way to catch up on Spiderman, along with reading summaries like Spider-Man: The Ultimate Guide.