I'm not sure about the European market but I do not think the prices will drop that much in the US once Vega is released. The prices may come down because there is more stock and less demand but I can only see a large price drop if one condition is met: if Vega performs directly between the 1070 and 1080.
In regards to the OP, I believe a 1070 is definitely worth it for 1440p 60Hz gaming or 1080p 144Hz gaming. Price is all relative to who you are talking to, some people have no problem spending 400 - 500 USD for the 1070 as seen by the current supply and demand while others would not even consider it unless it drops to 380 USD. The lowest priced 1070s are from eVGA and Gigabyte:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125875
http://www.newegg.com/Product/Product.aspx?Item=N82E16814487249
These prices do not seem unfair to me, 400 - 425 is right in line with a new 980 Ti (similar performance).