expect to pay 2, 3 or even 4 times the normal price if buying from a private hospital. The last time I got my bill, 1200 baht was for two medicines. Luckily the bill itemized the medicines so I took a photo and then handed it back to the staff and said I would leave the medicine. I went to my local pharmacy and paid just less 400 Baht for the exact same medicine.
Other private hospitals have cottoned on to this practice and so will not itemize the medicine so you may have to buy from them.