To a degree that is surprising; used to emphasize that something is unexpectedly different (often positive or negative).
このケーキはびっくりするほど美味しかった。
This cake was surprisingly delicious.
彼の成長はびっくりするほど早い。
His progress is astonishingly fast.
値段がびっくりするほど安くて、思わずもう一つ買った。
The price was so surprisingly low that I ended up buying another one.