This is one of those statistical kinds of problems. For any group of N competent people, there will be at least N * Log(N) valid opinions. For any group of N people drawn from the population as a whole, there will be approximately N! total opinions.
My opinion (for what it is worth) is that the Microsoft solutions will cost the least over time, unless your support and development time is free. MySQL and PHP offers a good non-Microsoft solution, at roughly 300-500% of the cost (but with no licensing fees). If you have the expertise available, PostgreSQL and Perl offer what I see as a better architectural solution with a higher cost of entry but lower ongoing cost. I can't offer any comment on Firebird/Interbase because I've never used them.
-PatP