I had one regular that I had been seeing for maybe 7 years when I realized the hourly rate he was charging was well below the market rate. I would usually book for two hours and I also paid for the hotel (he doesn’t use his home). So I just raised the amount I gave him (in addition to tip). At one point I just asked him what his current rate is and mentioned I had been assuming a higher rate for a couple years but I wanted to be sure it was enough. He said that he wasn’t going to ask me for a higher rate from me but he had noticed the increase and I was right with the amount I guessed and he appreciated my thoughtfulness. I didn’t want him to prefer booking someone else. I told him to just tell me in the future when his rates change and it has been several years more without another increase.
Whenever a have a repeat visit with someone else I always ask them to confirm the current rate and lately most of them tell me a higher rate nowadays. I confirm that still works for me but that avoids any awkwardness when we meet.