hi guys ............................few weeks ago i gave my phone for repair and they told that it was water damage..............now they are saying that they donot have spare parts thats why they have to replace the whole motherboard ..................................any idea guys what it will cost for repair.

any help guys..................................thanks

No idea. You might be better off just getting a new one, or holding out with a cheap phone til the N9 comes out.
Next time, turn the phone off as quick as possible, take out the battery, and leave it in a bowl of rice for a day or two.

I see you're from New Zealand. All phone repairers here cite "water damage" for any phone worth more than $600NZD (I know this as I worked for an insurance company for some time). Ask for evidence of said "water damage" before agreeing to any repairs.

any one know which part is to be replaced when phone is dead

First, the rice thing is a myth - you'll only end up with corrosion and water that evaporates at a rate that is no better than if you stuck it in your sock drawer. Note that all the guides say you should keep it in rice for a few days - you could also leave it on the table and achieve exactly the same result after a few days as well.

There are numerous guides and threads about drying wet electronics that don't involve rice, follow one of those, they are better. Drying should take no more than an hour or so, not days.

The N900 has a water ingress label - they'll just point to that if you question the legitimacy of their assessment.

Effectively, being in NZ, you'll need a new main board, this will cost you almost as much as the N900 itself unfortunately. If you're ever around Asia then drop in to one of the cell phone markets, someone will be able to repair it for you by swapping out only the dead bits.