It doesn't really matter how cheap the modem is, unless it has the traditional UART interface. With the UART interface, all you need to worry about (besides baudrate) is to ínterface the logic levels of the modem with the logic levels used by the 8051. With an USB interface, you must find a 8051 with a built-in USB host interface, or find any other solution that gives a host interface.

When looking at cost, you must look at the total cost, instead of the cost of an individual module. You mention a Maestro modem. But there are a lot of GPRS solutions out there. Which ones have you investigated?

The complexity difference between USB host and USB device is really significant. And the next problem is that the 8051 wasn't designed for the transfer speeds normally used by the USB interfaces on 3G modules.