Too costly? Is fixing your car's brakes too costly?
Mitigation costs too much? This is like saying 'fixing the dodgy brakes on my car costs too much, so I'm going to ignore the problem.' The consequence will likely be a wrecked car, and you in hospital, broke and suffering. Mitigation is expensive, but not doing it will be far more so.
Notes: For more variations and rebuttals of the "too costly" argument, see
As Midas repair shop and my 8th Grade Earth Science Teacher used to say, "You can pay now or you can pay later, but you are still going to pay anyway." However, with fixing your brakes and climate change it is so much cheaper to fix the problem now than later on. Furthermore, I also love the express that by doing nothing you have still made a choice.
I like the analogy which is easily understandable, and people don't like to spend money on a brake job, but they understand at some point the cost of the risk outweighs their desire to save the expense. And both have an indeterminate risk, yet with both it's understood risk goes up with loss of brake pad/higher GHG concentrations. Works for me.