Re: Purchasing plugins not availab...
In short 'No'. You cannot purchase any of Logic's native plugins separately. But, really the upgrade price from Express to Pro is extremely good value for money considering the number of extra content you will get, including jam packs, instruments, effects and extra applications. In fact the price of all the Jam Packs purchased separately would cost about the same as the full Logic Studio by itself.
You can purchase 3rd party plugins. For something as simple as the Multimeter, do search around the net for audo unit analyzers. You may find some that are reasonable or even free:
Hope that helps,