A PC scope, like most digital scopes, work by sampling the analog signal and then changing the sample voltages into numbers to be manipulated and changed into a graphic display by the PC. There are certain basic limitations that you will suffer. One of the primary characteristics of scopes is the bandwidth. If the bandwidth of the scope is lower than the bandwidth of the signal you are trying to see with it, then you will not see the signal exactly as it is, it will be distorted. This is bad. So the bandwidth of the scope is often used to describe the overall performance, even though there are many other factors. Let's talk about bandwidth of a PC scope.
The bandwidth you can get will be dependent mainly on the sampling rate you can use. This rate, in turn is dependent on what A/D circuit you use, and whether your PC software can collect the numbers from the A/D and then display them fast enough to not lose any of the numbers. I'm not sure, but i believe that the A/D capabilities of a typical Sound Card, for example, are only suitable for high fidelity audio, which translates to rates on the order of 30 to 100KHz. You should learn the Nyquist principle which rules that you typically will require a sampling rate twice as high as the maximum frequency you want to see (which is the same as twice the bandwidth). So, if you wanted a PC scope to display signals with frequency content up to 10 MHz (which would be a useful scope) you would need a sampling rate of 20 MHz.
As I mentioned, the sampling rate is something that drives your choice of A/D circuits and also is limited to how fast your PC software can go. The way that you interconnect the A/D into the computer may also limit the sampling rate. For example, there will be a maximum data rate that you can achieve using a parallel port. I'm not sure what the limit is, but it is reasonably high from the hardware point of view. However, if you access the parallel port through the Windows operating system you may find that the OS cannot go fast enough. Perhaps your program could access the parallel port directly for faster speed.
You ask:
"and how can the design be improved for the purpose it is being made, keeping in view the cost factor"
This is difficult to answer since you did not state the purpose of the scope nor have you stated the cost target. Can you add more please?