So i did some more research and talk to a recent visitor, and he confirm many Thermae girl prices are negotiable to 2000ST/4000LT, unless maybe she is super Chio or stubborn. You may be able to get even 'slightly' lower depending on circumstances (e.g. near closing time, too many girls, little customer etc..). It helps negotiate if you act confident and strong, but still friendly, normal looking, and appear a 'safe' customer. If you are old and scruffy looking you may just have to pay opening price.
I know many Bro's don't care and just accept pay the artificially inflated prices set by the girls, but for those that don't wanna get chopped,
there is no harm to a little friendly negotiation. Worst thing that can happen is she say no, at which point you can still accept, counteroffer, or find another girl. The key is to be friendly, and keep smiling like all Thais do. Thermae has a very mixed bag of girl quality with no QC, and for me 90% of the girls are not worth the inflated 5k set price they ask for. Most of them just average looking girls in a dress with lots of makeup fishing for a quick Jap Payday.