Mel,
The olde saying, I think, probably referred to the value of the potential crop from the colony in that particular year.
Later colonies had to be over-wintered for a crop the following year, so it is still relevant, in that an early swarm will provide a harvest but a July swarm is not so likely to. Nothing to do with over-wintering; if a beekeeper could catch an early swarm, it was almost a 'free lunch' for him or her. One straw skep (zero cost?) and 3 or 4 months later a dividend! No inspections, no real effort from the beekeeper!
So in todays times in terms of monetary expenditure: a nuc in July is a potential loss of a £130, plus all the time, etc. spent on it, if it fails survive the winter. A swarm will cost nothing, so is worth that £130 that would have been risked....
Regards, RAB