Why does coolant mix with engine oil?
Mostly in Two cases Coolant Mixes with Engine oil.(1) Blown Head Gasketi.e: intact head Gasket ensures integrity of coolant Channels and oil channels. Once Blown it may let Both essential fluids to get mixed at the point where is it blown.(2) Faulty/Damaged/Corroded Oil Cooleri.e: Oil cooler is an assembly employed in An automobile engine to Maintain oil's optimum Temperature, It is so constructed that oil channels and coolant channels are kept adequately close to ensure only Heat exchange can take place between them, Hence Oil would Dissipate It's heat using nearby coolant channel, but when oil cooler is not completely intact, Due to its construction and close proximity of both fluids let them get Mixed. Engine itself and oil cooler are the assemblies where both fluids are used simultaneously.Good Luck.
What is the approximate cost of fitting a new oil sump?
The "oil sump" is what the Brit's refer to as an "oil pan." The cost of replacement will vary greatly from car to car. Some are not much more trouble than raising the car off the ground while others require more work, up to engine removal. What did the shop quote? If you feel that is too high, take it elsewhere.