Sorry, this is so, so dull.
I've an oldish X9 rear mech (bought off here a couple of years ago). One of the jockey wheels packed up - they have bearings in which seems overkill, especially when they break and seize - so I bodged an old cheap shimano one on (probably off some '80s road mech). Really thinking I should buy a proper one but the cost! Should I buy a pair or a new mech given I can get a new X5 or X7 mech for similar money?