If you manufacture a wedge, which is just a mechanical interconnection to the PS/2 cable, it will cost about $0.25US. If you wish to build one in the lab, it will cost nothing, aside from your time, since you then simply strip a section of the PS/2 cable and splice into the open-drain-driven signal wires.

The mouse's logic will tell you exactly what you want to know and in a well defined, published, standard way.

If you want to do something else, that's fine. It's your project. However, you shouldn't claim it's more economical than to use fewer parts and an already-proven protocol over an unknown and, so far, unproven approach.

Give the standard way a chance. There's good reason why the makers of barcode scanners, mag-card readers, aftermarket keypads, etc, have used this method. Ready-made mouse-wedge cables should cost less than $10 in very modest quantity, and that's here in the U.S, where nobody cares whether things cost 1x or 10x the "reasonable" price. If they sell at $10 per unit in dozens then the manufacturing cost has to be less than $0.25, allowing for transportation, packaging, duties, commissions, etc.

RE