He said that a testing environment costs $300. If developer time were free, then yes, he would have agreed that his argument is invalid. Developer time is rather valuable, especially when considering opportunity costs.
If there are differences between IE and (Chrome|Firefox|Opera), then it will require additional developer time. Just testing for those differences will require additional developer time.
I don't want to demonize IE. Supporting any browser requires testing. IE has been especially reluctant to adopt web developer trends and needs to work hard to regain developers' trust. Supporting it isn't just the cost of a testing environment.