The actual cost of brining another aircraft type online is significantly more than the cost of the lease, the crew and the fuel. That first 737 or A320 also requires maintenance programs to be put in place, training of crews, increased crew costs, fleet management apparatus, insurance, "teething" expenses, etc etc.

Also, I'm curious where you got those figures from TD. Assuming they are accurate, thats a 40% increase in cost with no guarantee of being able to fill those seats.