Water gone in my lenovo 450 model laptop, whta i wnt to do to repair it , how much cost it will take.

Welcome to FixYa! Based on your question, Lenovo does not cover water damage when it comes to warranty. The cost will depend on how damaged your laptop is(a friend of mine paid $256 due to hardware parts of the laptop needing to be replaced) and I suggest bringing it to a licensed computer technician to avoid unnecessary spending.
I use denatured alcohol (not rubbing alcohol, Denatured alcohol.) If you have any signs of corrosion then you should wet your board with the denatured alcohol then use a tooth brush to clean it. Thats what I do and most of the time it fixes it. Aside from that you could seal the pc in an airtight bag with some rice. The rice will **** up the moisture. Hope this helps. Thank you for using FixYa!