You mean ignore the customers with spare cash to burn in favour of those with no (spare) cash?
Let's say old browser users spend twice as much money at your site as new browser users (typically mobile). Then the break-even point would be when you have twice as many new browser users as old browser users. Example:
IE 8 or less users:
-- 200 transactions per month.
-- Average expenditure: $80.00
-- Total revenue: $16,000
-- 405 transactions per month.
-- Average expenditure: $40.00
-- Total revenue: $16,200
In this scenario, it's probably time to focus on mobile-friendly sites/apps and dump legacy IE support.
(I'm assuming your org doesn't want to re-code the same app/site for multiple browser versions.)