A educated guess would be nice.
Define "educated guess"...
Educated, learned\schooled knowledge, reached through gathering information and/or x amount of experiance.
Guess, an estimation based on no/limited knowledge/information
Quite contradictional? Indeed
The right answer is: we don't know. We don't know how big the expansion is (let's call this the X- factor) and since developers usually use ye ol' "bigger dlc=more money", the price (Z) also depends on how many hours of content there is (Y) this is what we can come up with:
X*Y=Z
You should save up Z money. Happy?