Ok, personal experience....
We were planning to go to Moli D'Oli in Campredo.  I asked what they charge and they said there is no minimum or maximum weight and the price is 50 euros.
We had, we thought, around 110Kg and when we got there there was a queue (we hadn't booked a time) and we had other things to do.

Some friends use the mill at Camarles so, as we were passing, we stopped by and they weren't busy.  They muttered a bit about our low weight, claiming it is a 300Kg minimum, but said they would press them for us.  We had to give an NIE number and address and, in typical Spanish beurocratic style, went from one place to the other at least 2 or 3 times.

We ended up paying just under 19 euros - Camarles charge by weight.

You need to supply your own bottles and get all of your own oil.  Take more bottles than you expect to fill as you might fet a surprise!  We got around 33cl per Kg of olives.

Good luck,

Nigel