Hi there,
I have had a quick look at the site, it seems to be really extensive with a log-in area as well, do you intend for the log-in area to also be responsive?
It really depends on how the site is set up, if there is very consistent implementation of header/footer include files throughout the site and exactly how much needs to be on each page to accommodate this.
So, to give an accurate quote I'd really need to get into the code on your server and have a good look at how the site is structured, so the quote could be anywhere from $100 to several hundreds, on a basis of how long I think it would take, working at USD $200/full day -> as in, a FULL day of work, not a couple of hours dossing.
I have done work like this before. I worked full-time for 12 years for boatbookings dot com and made most of their site before I left two years ago, a year after implementing mobile responsive designs, and I also have one review on F/L from a chap in Thailand who asked me to work on his responsive site, which was mostly CSS work.
If you want to chat further we can, then we'd reassign the milestones etc.