A VT member wrote the following comment about visiting Napa:
Wineries do charge for tastings. by Herkbert
Many of the wineries you'll visit will have a tasting fee of anywhere from $5 to $40. Most will not charge you or will deduct your fee from any wine purchases you make, but not in all cases.
For example: Peju makes great wines that are only available through the winery. Their fee is $5.00. Silver Oak charges $10 and you keep your glass. On the other hand, a tasting at Opus will set you back $25. Only you can decide if it is worth it.