The sooner a bug/defect is found the cheaper it is to fix, so using the money to hire more qa people vs another developers, is going to cost you more time/money because of how many trips from DEV to QA.
Having said this, pair programing don't work with everyone, some developers don't pair well, they distract each other, spend all their time fighting, etc.
If you have developers that can pair program, it can be more then beneficial in the long run when you add in more maintainable code, lower defects so less time in QA, and most importantly if one of the developers get hit by a bus, you don't have to wait for someone to come up to speed on a project before any more work can be done on it.
If your developers can't pair program don't force them into it, all you're going to do is waste time and money.
