An in-line TDS meter for a nice RO/DI system will run you between $20-$30. I just bought an unbranded hand-held one that Hanna rebrands from the OEM for $5 on fleabay. Anything $5-$15 will work well for you if you spend a few bucks on calibration solution or known TDS water. I've had some cheap ones going for nearly 20 years. Still good as new. I calibrate only when I think about it. (Maybe once a year? Maybe twice?)
Right now you can grab a good one from Marine Depot for about $5, as they're going through their liquidation sale.
For pH meters, everyone is right. Those are gonna be expensive. Decent ones use probes that that remain in-tank. They range from $100-$250 and have to be calibrated monthly. Hand-held meters that are decent have to be kept in a storage solution and calibrated before each use - so I avoid them entirely.
But in terms of liquid test kits? Brand does matter and it depends upon what you're testing. Not everything is created equal and not everything serves everyone the same.
API pH kits may work well for the average planted tanker because they do well enough. But I keep sensitive shrimp species and like something with a more reliable range, so I tend to gravitate toward Sera. Only a couple dollars more expensive in North America but worth it for me. I prefer it over digital monitors. Same for gH and kH kits. It's all a matter of preference and what you're trying to accomplish.
P.S. Just say no to test strips. Tend to be inaccurate and the amount of plastic waste is bonkers.
