first of all, you need to separate 可and 下 here, they don't belong to one single word! It might be more clear if you put like this: 这可是 下了 血本 了。
Here 下 means spend/deposit. 血本 means original capital in business (a less fancy explanation) or (it costed them) an arm and a leg (a fancier way to explain).
Hope this helps!