Come on, 70 €uro for Skyrim + Dawnguard on Steam and you get how much content for those 20 €uro?
70 €uro = 86 $ Dollars
70 €uro = 55 £ Pounds
In other words ..
20 €uro for the Dawnguard is 25 $ Dollars or 16 £ Pounds!
Just so you know, there are over 10 million people who disagree with you. That's a respectable enough market share (well-earned, I might add) that your own personal concerns over price and your personal budget are not really consequential. Can't afford it? Wait until the GOTY edition is about 5 years old, it'll maybe come down to $20 or so total by then.
As to your 70 Euro... you do realize that the currency is, and always has been, over valued, right? The Brits abstaining was the best thing any European country did when Maurice Strong and cronies brought up the "world bank" thing. Anyway... how much is a pair of Levi's 501 (button fly, original pre-wash) in your country? Or, how much do you pay for a single movie ticket for a new release? 1½ hours for how many Euro? I'll let Skyrim's < 1 Euro/hour stand on its own now.