Over the years, how much do you think you've spent on probes, calibration solution, and what not. How much time invested in maintenance? Just curious.
Not much. I use the pH controller as a tool, not as an analytical instrument. I'm not so interested in what the "pH" of my water is according to the pH meter (because if I were, I'd have to calibrate at least every week, if not more frequently). More of a relative position from CO2 "off" to CO2 "on" As is, I calibrate about every 6 months. I've replaced 3 pH probes in 5 years. If they continue to calibrate, correctly and I don't see a noticeable drop, I keep using them (despite the conventional widom that you should replace them every 6-12 months).
I set my "pH" point to what my fish can handle, not what a chart tells me. This is why I like pH controllers because they allow you to easily make incremental changes versus a needle valve (or a $200 metering valve).
So, maybe $40 in pH probes (ebay), maybe $10 on calibration solution (don't buy it from an aquarium supply retailer), and probably maybe 2 hours over 5 years changing out/calibrating pH probes.