Wow, this a can of worms! I am going to put myself out there.....
Temp controllers (with PID) - Most cost is associated with the distribution channel and of course how many you buy. Take for instance a Fuji PXR3 a robust controller. Retails for about $120. I used to buy them for about $39.99 in lots of 25. Additional cost is associated with how many different types of probes that are compatible with i.e. K, J, T, PT100 etc. Most all of the controllers with give you similar precision (if set up properly) since in the espresso world we are only measuring in a small temperature band. I have done experiments with Thermocouples, RTD's and NTC's. Remarkably I have found very similar results using all three types. It really came down to the quality of the probe in all cases. I must note before I start receiving hate mail that I was only measuring between 195-205 and optimized all controllers for this.

Additional cost still is the support "some" companies give on their kits. This can be invaluable to the techs out there who would not venture to source their own components.

About your comment on pressure. While the relationship between pressure and flow is not exactly linear (depending where you measure) it is directly dependent. More flow=more pressure - less flow=less pressure. But again it all depends upon where you are measuring pressure.


BillC

karl_a_hall wrote:I had a cheap PID where the jelly buttons all broke within a year

Having worked with a not-terribly expensive thermal controller with jelly buttons, this probably goes to the intended mode of operation (set it up once and let it run for ten years) vs the espresso enthusiast mode of operation (I will make many temperature adjustments trying to get better and better results with varying coffee supplies).

I had to replace one controller on a 15 year old piece of equipment. We adjusted the new controller once or twice over the first week. It's just been loafing along, maintaining 500F (+/- 3F or so) for the last few years. (Not an espresso machine. That hot, the cooling flush would be terribly long.)