For a product in long run (may be 0.5-1 million users) is it good to use java instead of .net from the cost/profitability perspective. To elaborate more..consider web farm scenario in which one will have to purchase windows + Sql server + some antivirus licenses for .net applications. But for java it's almost free. What points should I consider?
PS: I am a .net person by heart
Go with what you know. With that many users price may not be important.
The first thing you should consider is the fact that you are a .net person. Java syntax is very similar, but it will take you time to learn (more importantly, become good enough to write an app this large) in a language you don't know as well. You should consider whether the savings are worth that time to you.
Also, keep in mind that there are many (legal) ways to get cheap (sometimes free) copies of the Microsoft software that you need, or the "lite" versions of the software that you need... which may be enough to get you going until your venture starts making money. So explore those before making a decision too.
I would expect bandwidth to cost more than software licences is you're reasonably successful - although obviously that will depend somewhat on the kind of app, and I haven't run any figures.
Are you just starting this up yourself, with little firm idea that it will take off? If so, I suggest you go with whatever makes your life easier to start with. If things take off, you'll either have enough money to pay for the licences easily, or you'll quite possibly need to rewrite the app anyway. Writing an app which provably scales to a million users is likely to cause you a lot more grief than writing one which scales to 10,000. Get the app out there early, get feedback ASAP, and you can do what it takes if it ever becomes truly successful.
License fee's cost less than a weeks pay for a development team.
Go with what you will be most productive in, and you'll save money.
Btw, WebSphere costs $1600 per CPU...Java isn't always free.
Whenever this topic comes up, I'm reminded of the part of yahoo that used to be written in lisp, and used lisp very effectively in its design.
http://www.algo.be/cl/TEE-lisp/3212824097309629/index.htm
Like others have said, it doesn't matter much what platform you choose, as long as you can use that platform well.
If you're talking about purely buying hardware it depends. You could run a Java web app for simply the cost of the machines if you decide to use open source software. However you could go with Oracle as your database and Weblogic as your App Server and then pay for it.
But most importantly, you say yourself you're a .net guy - so you'd have to have the intangible cost of however long it would take you to get to the point where you actually know what you are doing with Java. The syntax is very close, sure, but you have to learn which Java frameworks you want to use, which APIs you want to use, how to deploy to whatever App Server you choose, etc.
Joel wrote about this in Language Wars
How do you decide between C#, Java, PHP, and Python? The only real difference is which one you know better. If you have a serious Java guru on your team who has build several large systems successfully with Java, you're going to be a hell of a lot more successful with Java than with C#, not because Java is a better language (it's not, but the differences are too minor to matter) but because he knows it better. Etc.