When I started truing and building my own wheels, a number of books and threads recommended pinging the spokes and listen for the note made in the sound to understand their tension. I found this pretty frustrating and not particularly successful despite being in full agreement with the theory, probably due to inexperience and poor hearing.
I found it much, much, much quicker to use the Park tool to get the spokes to tension. Over time, it became easier to detect spoke tension by feel. Now days I can usually detect an offending spoke by working my way around the wheel pinching the spokes systemically and I really only use it as a sense check.
If your asking the question, buy it. I thought it was expensive at the time, but I've also used it to restore and build more wheels than I can count - so, so so much cheaper than using shops in the long run. I've never looked back on spending $$ on tools to build and maintain my own wheels.
But shop around internationally on line. I found UK stores would usually send goods to Australia far cheaper including shipping than what they could be purchased for here, including from local online stores. From memory, ribble.co.uk often had cheaper Park Tool prices than CRC or Evans Cycles.
I do recall seeing plans online previously using harbor freight gauges and a 3d printed holder mimicking a current design if you were interested in saving some cash? just cant remember which forum it was on off the top of my head.. let me know if you are keen and I will trawl to find it again.