Certainly with sailing compasses, repairing them once they start losing oil can be throwing good money after bad.
If you're leaking through minuscule cracks, Capt. Tolley's Creeping Crack Cure can be used - in essence it's just a very very thin glue that penetrates and seals. You can easily make a really big mess though.
If you've actually cracked it properly, you'd need to do a proper windscreen repair job on it - using a high speed drill to make a hole beyond the end of the crack to stop the propagation, refill the oil, then sealing it all up.
It's not really surprisingly the cost of repair - I'm sure they'd just drop a new compass unit in, and the rest of the instrument is just a hunk of CNC'd aluminium. Its the actual compass bit that has the jewel bearing, calibration and micro scale etc.
And considering your compass will be demagnetising with age, and the jewel pivot wearing an uneven hole in the card (+ any damage from transport once the oil leaked out and the was no longer neutrally buoyant), you should probably accept this as an excuse to possess a shiny new one.
We (Imperial College CC) smashed a Suunto clino of ours on expo last summer, it makes a lovely object d'art. The crack went right over the metal pivot arm, and the clino wheel itself is now wonky and wobbles like a top as it spins. There's also grains of mud (swilled around by the departing oil I assume) covering the optical window. No real chance of repair.