The sale that was on last week had JFK for about £450 in WT+ and £230 in WT for midweek travel in the lowest season; I got a handful of WT+ tix in the £550 season (plus £15 weekend supplement in one direction). That was about as low as it ever goes, with current tax etc. rates.

That sale was exceptional for offering WT+ at £100 extra each way. The normal premium is £200 each way over the lowest published economy fare, so if the lowest WT is £370 return, you'd expect to see WT+ at £790 return (because there's also £20 extra tax).