Why does it cost $12.99 and 12.99€?
Only £10.49 in the UK, using current conversion rate it should be £9.98 - having seen the Patch notes, all the new content is well worth it...
I see. So it's 70pence above the Dollar price in the UK and 1,40€ above the price in the Euro zone. Is there an explanation for that?
The EUR and GBP (and SEK, etc...) prices include the value added tax (VAT). In the EU, consumer prices must be given with taxes included.
The various VAT rates are set on a country-by-country basis. The rate applied to most items, including computer games, is the general rate, which is the highest one. E.g. 24% general VAT rate for Finland means that the amount of tax is 0.24 times the tax-free price of the good.
Companies within the European single market (EU/EEA except some regions) are also required to carry VAT from mail order/Internet sales to another country within the single market if the total sales to that country exceed a set limit (set by the destination country), or if the total sales the previous year exceeded the limit. The tax is paid to the destination country, following their rates.
So even though Steam's European operations are carried out outside Finland, Steam is required to carry VAT from sales to Finland, since the annual sales (tax-free) exceed 35,000 euros.
Having taken this into account, the math in this case becomes:
The US price, without tax:
12.99 USD = 12.03 EUR
versus
The European (Finnish in this case) price:
12.99 EUR (14.03 USD), including VAT 24%
VAT 24%: 2.51 EUR (2.71 USD)
10.48 EUR (11.32 USD) tax-free
So actually the Finnish tax-free price is 1.55 EUR (1.67 USD) cheaper than the US one! Some of this has to do with the publisher setting different prices regionally.
One thing to note is that (as far as I know), there is only one price for the entire Euro zone (that uses the euro currency), regardless of the VAT rate of the country, so the amount of tax and the tax-free price will vary accordingly.