Having studied several languages, I think the biggest difference in difficulty is in how long it takes to acquire basic proficiency. For English speakers, it's obviously easier to ramp up in Spanish or German than Chinese or Korean, because there is much more vocabulary and syntax in common.

However I think putting a number on the difficulty of becoming a fluent, capable speaker of any foreign language is much more difficult. I'm not sure if it's any easier for an English speaker to become an idiomatic, witty, eloquent speaker of German than Chinese.