To check the transaction costs you will pay to make a transfer, click on Send next to the account you want to send funds from: 

On the next screen enter the amount you want to send and click on Make a transfer.

The transaction fees will be shown to you on screen in milli DOT

If you want to send all of your funds out, you would have to deduct the transaction costs from the total amount you want to send, as otherwise your transfer will not go though due to "insufficient balance". There is also an easier way here