Amount of coins in your wallet will not change (if you don't send/receive coins) but the value of the coins in usd is constantly changing. So what you see is the current value. My guess is that this transaction was so expensive because it was bank transfer which is costly by default.
so my question is what is the most economical way to buy bitcoin as i came to know from the above post that bank transfer is a costly one.
I would say
https://localbitcoins.com/ is probably the cheapest way to buy bitcoin. You can also try to find someone who will sell you a paper wallet with the amount that you want, in this case you don't pay any additional fees, you just need to swipe the paper wallet to your own wallet and funds are there.