scrypt is deliberately designed to use a large amount of memory when calculating hashes, which makes it much more difficult to parallelize using a GPU (which don't have much RAM available), hence making it much much slower to attack.
scrypt cracking is still embarrassingly parallel even if it's hard to run on a GPU. I understand the term "asymptotically more" to refer to big O notation, where constant factors like that are ignored.
Well it's hard to state what exact n is here, but you'd probably be increasing both the calculation time and memory requirements at 2^n, so you need roughly 1/k of an entire computer to calculate even as Moore's law marches on and factors increase, but PBKDF2 parallelizes more and more.