We have to understand that there is difference in a specific objective software tool and a complete process control system.
For example a small software where you can index library books and issue them to uses may cost around 15000-25000, while somethig like a elibrary with Online Public Access Catalgue (OPAC) and erequisitions ets would jump to a scale of a larger library solution reaching to a range of 5-10 lakhs.
Similarly your basic requirements for maintaining a question bank with well classified question content are well fulfilled by the standard QPG software.
While requirements like ....'the login of users generating their own qpapers and controlled by permissions to edit' .....they make the solution jump from Standard Qbank Software utility tool used by Schools/Coachings to Advanced Qbank System used by universities.
This login based question paper generation system, no longer remains like a portable Qbank software tool that a user can carry independently on his laptop.
It takes shape of a network integrated complex browser based system with ability to register users, map them to classes and subjects and allocate varied permission to each user.
Accordingly the proposal price of standard Question Paper Generator will jump in multiples for base price of advanced question banks system.
Then such a large system integrated with university server and netowrk also has some custom implementation needs for which charges are additional to base price.
Generally this type of solution is beyond the commercial feasibility of a private school or coaching institution.