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.
A developer is not a testing environment, and the developer price is paid for testing in Firefox, Chrome, Safari, Opera or what have you. MSIE is no different on that front.
Of course a developer isn't a testing environment. Developers don't cost a fixed $300.
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.