A wireless apparatus for this application isn't rocket science however, the design, construction, testing, packaging, etc costs are substantial initially. I don't think I'd ever be able to come close to the cost of any consumer electronics - even something like the wireless - unless I put zero value on my time and had some unusual access to parts, supplies, equipment, etc.
I've done the make vs buy comparison many times and "buy" always wins if cost is the only consideration - and as long as the comparison is equal. If there is some feature I need/want or unusual function then I might have to make what I need because I can't buy it. I do make plenty of things but it has more to do with my interest in learning or the satisfaction/sense of accomplishment.
That's not to say that there isn't some opportunity in making wireless equipment for guitars. More than once someone with an edge or desire has found a better, cheaper way - and they were driven by the feeling that the stuff just should not cost that much. Sometimes they are right - sometimes not. A careful look at all of the costs, including time, might reveal an opportunity - or confirm that a do-it-yourself approach won't be less expensive.