External serial modems tend to be more expensive because they are a full hardware modems, and don't hardly need a computer to do what they do. They process the data and pass it to the computer, the computer itself need barely lift a finger to download anything.
Not to be confused with cheaper external USB modems, which are strictly winmodems, and only act as an interface between the phone line and the computer. They have no processing capability of their own and use the computer's resources to do everything.
Everything about them is simulated via software, and I've had the poorest of speed and luck with them.
Sorry, but the good stuff costs...usually over $70 US.
Right now I'm running a USRobotics 5699B that connects well (36k in the far far boonies), but has random issues with faxes and some downloads. It was only $20, so ya get what you pay for. The winxp drivers work better than USR's own drivers...strange.