No, thats heaps cheap. They go for between $350-$500 from what I have seen depending on the quality of the job done.
Not too much at all mate. $300+ on 'trade' (send your plenum, they send you a polished one) in the UK. So that's not even 'buying' a polished plenum really; just paying to have one polished! I'm SUPER tempted to buy this, but it's more than I need right now, and the wallet is stretched enough, as I just bought a car hoist to make it easier to do some work on my Z