Seems the generally accepted value is 1p per Avios whereas that values your 73k Avios at a shade over 0.5p each. But if you've got loads of the things to burn then why not and £400 is a decent chunk of spending money.

For the OP's conundrum if I'd earned that many Avios flying in Y I'd be making damn sure that when they're redeemed it'd be into anything other than that same cabin. It seems a perverse outcome that the reward for a whole stack of economy endurance is enduring yet more economy.