Ah. I was thinking of a bridge in which the railway goes above the read, hence my comment about building another one alongside it.
But I would imagine roughly the same logic ought to apply if the road goes over the railway: Whatever the engineers did to build the first bridge, with its single-track railway, they could just do the same thing again, say, 5-10 metres away, to create a separate bridge for the 2nd track, and at a first guess that should be a similar cost to the first bridge. Making it say 5-10 metres away means you're not building right next to a live running railway, so you don't get those associated costs.
There may be other complications. Perhaps the 1st bridge was in the optimum location so the 2nd bridge will have to go in a more awkward location as far as the road alignment is concerned, but it would seem surprising to me if that sort of thing is likely to increase the cost by a factor of 10.
Obviously that's still much more expensive than just building the first bridge wide enough for 2 tracks in the first place would have been, but it suggests to me that it ought in most cases be possible to accommodate a 2nd track for (roughly) the original cost again rather than 10 times the original cost.