I enjoy cleaning tack, so I wouldn’t pay… but I would charge. 
To answer your question, I would do a flat rate based on item, not based on condition. Figure out how long it takes you to clean each at their worst, and then figure out how much YOU need to make it worth your time/products. That’s your flat rate charge.
I used to have a side-hustle doing tack cleaning, mane pulling, grooming, etc for a big competition barn. I was charging $20 flat rate for clean/condition bridles and IIRC, $35 for saddles/girth. That was in early 2000s, so factor in inflation and I don’t think the $30-65 range is necessarily out of realism.
Cleaning/conditioning harness and/or western tack is a whole different ball park and I would expect to pay much more. Especially harness – I did that once and after the time, work, and products used would say $200 was way more in line with my time.
This is assuming you are doing a full clean: disassemble, wipe down, clean, wipe down again, condition, finish with product of choice, reassemble. That takes time, attention to detail, and I’d even argue a little skill - as you are looking over hardware, observing for any structural issues or safety, and then putting it all back together. Putting a harness back together correctly is a skill, so is a bridle.
I remember back then my services were more expensive than other teenagers at my barn, but I was also using Akene, Belharra, and other high end conditioners. High quality soaps and conditioners are expensive and should be built into the base rate.