to my knowledge, all registrars pay the same cost to register .com/.net/.org domain names in US$, so any big fluctuations in currency would no doubt be reflected and passed on in the prices offered by european registrars.
They could absorb small fluctuations, but I would imagine larger fluctuations would result in price changes.
This is true on .com, .net, .org, .biz, and .info. Similarly .uk will be governed by the pound sterling. This is true regardless of what currency you use to pay....what do I mean by governed...let me expound.
Let's say you found someone set up in Euro at a current exchange rate of 1:1...and they purchased for $8, sold to you for 10 Euro.
Now, there are only two choices going forward. Either their Euro rate will float....
or they will periodically adjust it.
But, lets say the rate goes to 1 Euro =$0.80....there is no way they would stay at the 10 Euro price, because their cost would now be 10 Euro...so, however they go about it they are going to increase you to something like $12. If the currency goes the other way, they may or may not pass on the benefit to you. Only if the reseller were charging a higher price...like 15 euros....could they afford to absorb any negative currency revaluation.
But, the underlying basis of all the pricing will have to be in US$ because that is where the domains originate and the way Network Solutions prices to the registrars.