ruinous
adjective
uk/ˈruːɪnəs/ uscausing great harm or destruction:
The bank had capital to spare, which they lost in a series of ruinous takeovers.
be ruinous to sth The proposed bill is ruinous to the home building industry.
ruinously
adverb
It is ruinously expensive for local e-commerce companies to accept credit cards online.