VTA's apparent cost seems high, but not totally out of line. Based on what I've seen of work in Sacramento (a much lower-cost market for many things) and in a few articles:

Just Laying Track (with power supply, wiring, etc. for light rail) in flat terrain including maybe moving a lightly used frieght line to the side of the r/w and occasional grade crossings but no bridges -- $20-30 million a mile

Add some basic stations with parking lots, or more substantial grading/clearing, OR a couple of bridges -- $30-50 million/mile

Fancier stations (perhaps because you need them to keep the neighbors happy), some bridges or other structures (retaining walls aren't cheap), more utilities, etc. -- can easily pass $60 million/mile

Do it in downtown streets, especially in an older downtown where the location of utilities is mostly guesswork (and of course allowing for the odd archeological or historical site and some random hazmat) -- $60-100 million/mile easily and $200 very possible. Even if you're just building a streetcar line in a downtown area don't be surprised to see prices like this - you can study 'til the cows come home but you never really know what's in those streets until you open them up.

Then there's the current poster child for excess -- SF's 3rd Street-Chinatown light rail (!) subway -- $1 billion/mile. Hopefully that's an outlier that won't be approached by any other project for a long long time. Even BART seems to be running much less than that (though without a lot of serious tunneling) on the Warm Springs and Berryessa extensions.

Given all that, ~$200 million/mile for the VTA extension is on the high side but not ridiculous.