I believe that Murray says to Robert at some point during season 3 that the 4th Earl of Grantham only 'saved the estate by dying.'
Assuming that Robert is the 5th Earl of Grantham (which seems to be a pretty fair assumption) does that mean that the money Cora brought to the Crawley family was only 'tied up in the estate' once her husband Robert actually inherited the title and estate? I.e. the death of the 4th Earl meant that Robert inherited and his wife's money saved the estate from financial ruin.
Does this mean that before Robert's father died (which we can safely assume was after Robert and Cora's wedding) he and Cora would have been an incredibly wealthy couple who were able to live independently?