I just wrote a huge response on this topic but after re-reading some of the replies, I'm just going to keep it short & simple...
Is it worth it to master 2-3 songs and leave the rest of your album as it is? No! Regardless of what you're being told, that makes your project sound unfinished and makes you look unprepared. You NEVER want to leave that impression on people.
If you can't afford to do the whole project the right way, WAIT UNTIL YOU CAN! There is no rush to put it out, especially if you don't have a huge buzz.
Another option is to go with someone who charges less for mastering. Brian is good and if he accepts your project, it will turn out nice but Brian isn't the only cat who does his job well. Take it serious enough to do a little research.
NEVER, NEVER, NEVER, put yourself in a situation where you have to make excuses or explain ANYTHING in regards to your music!