It really depends on what the client requests. If the specs say that the site has too look different on desktop, mobile, tablets, etc. and it’s clearly specified how different it should be, many companies still consider every breakpoint a wholly different site.
Because of that, all the costs go up: extra design, extra development, extra QA, and so on. It’s not uncommon to charge so much more, but I do think this will change in the future when responsive designs will get more “mainstream” and default.
So yea, 4500 does seem ridiculously expensive but it probably has to do with the fact that this company doesn’t really have their responsive skills properly integrated yet.