How it is cheap? Didn't you think about the software overhead (time and efforts) to make it work?

There are lot of multiport RS-232 cards available and I don't think the cost of ready made card is way too high to get involved in the development of such a system.

This scheme assumes that PC software has to be written. What if it is a ready made software for that particular piece of hardware (the device which Ravi needs to connect to PC, 10 at the same time).

Also if the devices in question exchange lengthy data strings and half of them start transmission simultaneously, what will happen to (already RAM starving) 8051?

So the only practical solution is to use a multi port RS-232 card if the devices and PC software is ready made.