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]
(..) Skin? You're saying Dean's dating my sister because of her skin? For blood, I mean. I'm just--I'm just saying it could be a contributing factor. Hermione's got nice skin. When you say it, as skin goes, I mean. I've never really thought about it. (..)2009 Harry Potter and the Half-Blood Prince