Just to expand on what nink said, if you have your hours for the CPA and have a job I see very little reason to get a masters.
If you don't have a job lined up and don't expect to find one before you graduate, then the masters is probably the better option over unemployment.
I know a few big 4 offices (and I'm just assuming you're looking at big 4 firms) give slightly higher starting salaries if you have a masters (maybe $5k higher) but this depends on the firm, the office, and the practice so it's not really something you can count on unless you know where you want to work beforehand. Not to mention $5k extra doesn't do much when you consider the factors you already listed like opportunity cost and tuition cost.
Some might argue that later on down the road the masters might look nice when you're looking at getting a different job, but the extra year of work experience (assuming you start work straight out of undergrad) could be just as, if not more valuable than the masters degree. And you can always get an MBA later on if you feel that a lack of a graduate degree is holding you back as far as job opportunities.
Bottom line, as nink suggested - if you have a job and the hours to sit for the CPA, I'd skip the MAcc.