What do you mean by cost of living?

Inflation would be one way to frame it, but it's basically certain we wouldn't get net deflation for trumps term unless some insane catastrophe happened, and even then that might just cause high inflation rather than deflation.

Real incomes is more uncertain, but even though I think he's gonna be pretty bad for the economy, it's pretty hard to cause net lowering of real incomes across your term.

Does it need to be attributable to him or just happen at the same time?