Given how many factors feed into transactions it's hard to say from that graph alone, but if you compare it to other graphs like 'visits', 'fame' and 'satisfaction' it should give you some insight as to where the problem is.
I've found price setting has very little impact on transaction numbers until it hits a certain threshold then demand completely collapses as per your graph. But the threshold doesn't seem to be fixed but rather moves about in a fairly narrow band. I once saw something like your graph when I set drink prices to $96, which worked OK for a few months until demand suddenly collapsed for no apparent reason. I immediately dropped the price to $48, which brought demand back up, then inched it back up to $90 and left it there with no further problems.
(BTW, have you got a manager looking after inventory for you? Have you told him/her to only restock with top quality stuff? Could be the average quality of your inventory fell below a critical level.)
Thanks for considering.
Visits are more or less parallel to transactions, fame stayed about level/slow rise, satisfaction has indeed gone down massively (74% last month to 50% this month, though only day 3) though I don't understand that either since I have made no changes whatsoever to my lineup except for training them. Prices are well below the range you mention (drinks are at $9.50). Other than you I have noticed an immediate drop when I raised them, see the second slump in my graph above. Even raising the admittance fee from $5 to $6 (which is what all other clubs in my city charge) seems to have upset my clientele.
And yes, I have two managers looking after my inventory but I also occasionally watch it myself (they both have other duties, so every now and then especially alcohol drops low), max quality (looking at my stats, food only has 9.97 on average, I possibly misclicked on one order, but that alone should not kill me, especially how low consumption is when compared to the 10.0 drinks).
But may main befuddlement is not so much "how do I get out of this" since this playthrough has more or less run its way anyway, but "how comes a sudden - not slow but immediate - 85% drop when I have changed nothing in months"
For transparency's sake I should mention however that I did change the room file (any bugs encountered when fiddling with the data are on the fiddler) to include an idea from this thread (have the 1 person rooms filled by 2 each so they don't stay unused. Even though the pool really seems useless), but again, that's been going on for months without change so is unlikely to be the cause.
There seem to be a few NPCs confused about their sexuality in this play through, such as the gay woman who dislikes homosexual sex and hates eating pussy but loves large penises. In another game I came across a sex worker with several years experience who was a virgin. She must have disappointed some clients.
Generally speaking employees with several years of experience seem to be weak though. 16 years as a waitress and a service attribute of -34 seems off to me. Even though I know some real life employees who seem to fit that bill ...
The virginity might be explained by her only offering special services
![Wink ;) ;)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
- but yeah, according to fluke they should usually come with the proper attributes.