Payment is in Siacoin. Whatever 1 SC price is doesn't matter. If Siacoin price goes higher -in BTC, for example- then you will get less storage space for the same amount of Siacoin, but you will still pay the same amount in Siacoin.

Storj makes it different: they charge the uploaders in USD and then pay -once every month, cough cough more or less in that period cough cough- to the people that host the files in SJCX -or since this week, in STORJ-. Again, if I have to buy USD because my bank only holds EUR, I will still to take into account the EUR/USD value.