How much should I tip my hairstylist?

I have finally found a hairstylist I like however he is very pricey to me...usually what I get is a wash and flat iron all other hairstylist charged $30 this guy charges $40 this is only pricey to me because I am a full time student with a part-time job and I like to go every two weeks...what is you alls take on how much I should tip for a shampoo and flat iron?