How can I include "compute_price.pl" into the main cgi script above, so that it prints the total price?

Usual answer: just make compute_price.pl into a module and use it from you cgi script, and possibly from a simplified cli version.

How can I also show the "die" version into the cgi-script when parameter given is incorrect. Just like in "compute_price.pl" ?

Just have a sub or method return, say, undef and have the main program display an error page if so.