Well they might recognize if they have more options / more countries to choose from on the join page.
I don't think that many discuss how much they pay for porn with their peers though, and any place where they can discuss it is just wrong
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I assume it's the right way to go though, make a fixed price in different currencies, adjust the price to the market.
At least it sounds fair to me, as far as it should - firstly enable you to get more sales from weaker markets and secondly - it doesn't screw the surfers from the countries with a very unstable currency.
It would be interesting to hear the experience from the billers though.