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]
(..) Martin, your customer. Uh, can l help you ? Just, um, incidentally, uh, why... are you wearing that ? Combination of factors really. Uh, no clean clothes. There never will be, you know, unless you actually clean your clothes. Right. Vicious circle. (..)1999 Notting Hill