At least a few runways are divided by length. For example, once at Boeing Field (BFI) four general aviation light aircraft were cleared to land approximately at once on two runways. My VFR landing clearance was to land in the first half of runway 14R while another aircraft above and slightly ahead of me had to stay at 500 AGL(? I forget the exact number) until the threshold and then land on the second half. The left runway had something similar occurring at about the same time.
Having irregular width runways doesn't help this much. Also it is difficult to imagine that the non-ends would be graded any differently. In the overall project of constructing a runway, it is highly likely that the land preparation, utilities, and grading are like 80% of the cost of the surface. A forum exchange breakdown of a Canadian 7000 x 100 foot runway is:
$ 250,000 geotechnical analysis
$ 4,000,000 gravel (60 cm of 7.5 cm minus, 25 cm 1 cm minus)
$ 2,500,000 labor and equipment rental
$ 100,000 quality testing (compaction, etc.)
$ 2,000,000 asphalt (15 cm)
$ 200,000 lighting
$ 10,000 painted markings
$ 9,060,000 total
Would making the shape narrower increase any of those costs? Probably. Certainly less materials should decrease the overall cost, and maybe labor could be economized.
For strong crosswind landings, I usually use all of whatever width there is!