It is a little hard to tell from the scan. You might try putting only the stamp in the image and make sure the black background goes all the way behind it and can be clearly seen on all four sides.
But from looking at it, I don't think you have the line perf. I think you have the comb perf version of this stamp. Some of the corners of the line perf stamp tend to be more jagged while the comb perf stamp is better defined on the corners.
The Commonwealth Catalogue lists the line perf as 14.25 on all sides, and the comb perf is 14 x 13.75. You need something like the Gibbons Instanta Gauge to measure this. The gauges with the circles won't show the detail you need to see.
However, assuming it is the line perf, a used copy sells for around $50. If it is the comb perf the typical price would be about $2-$3.