It is entirely personal. If you will never travel again, any use of miles to reduce the cost of your last travel is worth it.

If you expect to book a rental car or a hotel at any point in the future, they are redeemable at 0.67p for a hotel/car hire booking so that has to be the minimum.

Beyond that, I'd suggest that somewhere between 1p an 2p should be achievable without too much difficulty - 10p sometimes happens but I wouldn't count on it!

So on the basis that you value an avios point at 1p at least, you would expect to get £500 off for 50k avios (50,000 x £0.01), etc. On a one-way to the US then that will easily be achieveable, but then you may want to return at some point. If you're travelling non-stop LHR-JFK in February in economy return, the taxes and charges can be more than the actual price of a ticket...