OK, sounds good. Go easy at first though, sometimes that "20%" will get ya. But the $900 is still crazy. I take it you don't do the DIY thing?
PS (I'm PS crazy tonight): If you plan on keeping the car for quite awhile, then consider the new tube if it does end up being plugged. If maybe only a couple more years, then you persomally would probably do OK with the speedo-cable trick.
: And also it's possible it's not the tube OR the valve, OK? There's a switchover valve, which needs a vacuum source, and the switchover valve is controlled by the engine control module, etc etc, so it's not a case where it "has" to be the valve or the tube, just a heads up.