It depends on the company, the change, and the timing.

If you want to swap rooms around etc that doesnt effect floor space size its can usually be free, if you are making rooms bigger etc, then while there may not be a cost to change the plan, the build cost would go up for the extra materials etc.

Also some builders will charge you a fee per change to the plans after they've all been signed off on and materials "supposedly" ordered. (Funny that, builders will tell you they order all your materials in advance, then when its time to put in your floor tiles they've been 'discontinued...')

PM me if you want some more info