A 'ream' of paper is, to my knowledge, 500 sheets.
I haven't seen the advert to which you refer but usually photo paper is advertised by the pack of 20, 25, 50, 100 etc.
I don't think you have been ripped off. To me £8.35 per 100 sheets seems like good value. I use a fair amount and cannot recall paying as little as that.