How much do i charge for a "Wholesale Booking System"

Hi.
A Travel Agent from uk wants a website developed the details required are in the attached file.
I am thinking of developing it using wordpress.
I am from India, and a website like that should cost around $3,500 or more here. I have been charging my customers around $600 and having a problem later with having to do more than I get paid for.
I want to charge him minumum $2,500.
My question is would that sound reasonable enough? how much would you have charged / paid?

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