I don't believe they've said anything about pricing at all.
It may be worth being prepared for the idea that the purchase price for the expansion may even be cheaper than $20; remember that this isn't strictly payment, but instead "Donate $X and get a free Y"
I bring it up because I suspect the finished game won't cost much more than $30 to purchase - if not cheaper - and the expansion cost being two-thirds of *that* strikes me as a little high.
That was exactly my reasoning to not increase my pledge for the expansion but do a more substantial increase to reach an higher reward tier instead. I'll buy the expansion afterwards.