My wife and I are planning a trip to St John, staying at the Westin. Is St John and St Thomas as expensive as the Bahamas, when it comes to eating and drinking?

Yes.

St. John, and lesser extent, St. Thomas offer a range of prices for most everything, especially food. Depends on your requirements for POSH. A friend and I (we both live here) had a lobster each last night and paid $35 medium, his: $39.00 for large. We could just as well have had a coconut chicken dish, exquisitely prepared for $18.00. Good sides too.
You chose to stay at the Westin, not my favorite for the price. You can stay on St. John in a park campground or a small guest house or villa for less "Americanized" lodgings...anything after Easter is less, too. Hope this helps. Bottom line: ask a local.