As is the case with most things motorcycle related, it's all about how much personal risk you are willing to assume. I've plugged a tire and then rode it for another 2000 miles without any ill effects. A lot of people (including EVERY dealer) will tell you not to patch it, it's not safe. My personal opinion is....it depends. I wouldn't take a plugged tire to the track, but I don't have a problem with standard street riding/commuting on one. If the puncture were a large hole, or were in the side wall it might be a different matter. But a slow leak from a nail or small piece of debris isn't going to cause a catastrophic failure under normal riding conditions. If you are going to do a lot of craziness where the tire is experience unusual stress (heat, abrasive or otherwise) then I'd recommend going ahead and replacing it. If it's been sitting flat all winter, I'd recommend replacing it. If you just caught the nail yesterday and it went flat overnight....go ahead and plug it.
Again, all of this is about how much risk you are willing to assume. If you aren't comfortable with a plugged tire, then get a new one and don't think twice about the money. Peace of mind and confidence on the road are worth far more than $150.
And that's the gospel according to shaggy....now forget what I just wrote and go do whatever the hell you want.