I recently bought a set of reusable stainless steel straws. They were expensive, but have a timeless "design" and have a build quality that gives the impression they will last a lifetime. When dirty I throw them in the dishwasher with the other cutleries. They don't take any space in the washer so cleaning them is essentially "free". (Making abstraction of the ecological cost of a dishwasher.)
Honest question: How many plastic straws do you have to save to offset the CO2 cost of building a steel straw? Also, are paper straws really better, CO2-wise?
As the other poster said, this is very hard to calculate.