Cost was the 'decisive/deciding/determining factor' in their decision. [=the most important reason for their decision]
The 'deciding factor' was cost. [=the decision was based on cost; the least expensive option was chosen]
Costs increased/decreased by a 'factor' of 10. [=costs were 10 times higher/lower than they had been]
(..) I knew. You did! Oh.... I always figured you just thought I was Monica's geeky older brother. I did. Oh. Listen, do you think- and try not to let my intense vulnerability become any kind of a factor here- but do you think it would be okay if I asked you out? Sometime? Maybe? Yeah, maybe... (..)1994 TvShow: Friends Title: The One Where Monica Gets a Roommate Season: 1 Episode 1