Not been to Nana yet but I hear it's quite amazing and definitely the place to go in Bangkok. Price wise (based on Samui), I'd say expect 300-500 THB Bar Fine and a realistic average of 1000-2000 THB 'long time' for the lady. It can depend very much on the individual lady and what they think you are willing to pay.
I hear that Areeya is 10,000 THB/night - if you can get her to go with you! Worth every Baht in my opinion.