Would the USA let you in without a ticket to return home?

Anyway assuming you get one before you go, as it's now less than T-21 i'd have thought the price you are seeing is reasonable.

Assuming you used 25k miles for a one-way, you might want to try part miles / part cash to see if there is a cheaper complete return ticket and if so cancel your original and go for that.

Other than that or trawling expedia etc for a cheap flight on another airline this might be the going rate this close up.

Added : or as HIDDY says, cancel this and buy a paid up return for less and keep the miles, showing as £510 return incl taxes etc leaving midweek next week, returning 23rd