Hi,
As Patatas already pointed out, you need to be very serious and dedicated to make this work. Most definitely you need a sound business development plan.
I roughly estimate that if you have a dedicated team e.g. 2-4 core developers that it will take you atleast 6-12 months and approximately between $100k-150k to have something decent to show, if you start from scratch.
And than there's all the legal stuff you need to go through and deal with.
But you can take the easy route and buy an exchange script for a few thousands dollars
I have wanted to started my own bitcoin exchange or buy an existing one but after some research I felt it would be to difficult and time consuming to pull it off on my own.
Good luck in all your endeavors.
This numbers varies so much from country to country. They are maybe right if Op is from US but in EU, SA or Asia reality of development of bitcoin exchange is totally different.
You can do it much cheaper, laws and regulations are different too.
Where do you saw 'exchange scripts' for sale? And second question: will you trust exchange which is based on reused and bought script?