Depends on how valuable your time is. People justify spending an extra £5k on an air ticket to give them 10 hours of "working" time, which is £500/hour.

Spending £3 and saving 5 minutes on the tube would therefore be worthwhile.

(The big question is "when is the next hex?". If you've just missed one, take the tube. If it's in 5 minutes, take the hex)