In the future, it would be a good idea to ask your designers that your site not be so static, so you don't have to touch 150 files just to make a simple change to the master template width. Most sites rarely see anything beyond a 3-tier template structure, consisting of a home page template, main category landing page template and a content page template. Any differentiation within each of these templates (i.e. if you have a different look and feel for multiple, level-2 product pages) can be addressed easily with includes that pull in the proper UI "wrapper" assets that you need for each respective page.

Had the foundation been laid more properly, this should have been a two hour task for the template tweaking, and approx. another 8 hours for the graphics rework if any.

Looking on the bright side, the project fee they want is not steep at all, so you should try to work it out amicably with your designer/programmer, and move on with business. I'm positive when I tell you that the shrinking 800 x 600 user segment is used to dealing with far worse than the horizontal scrolling bar creating an eye sore for you. When you redesign your site again, consider using a more dynamic template structure.