aren't any payment required for those platform for easy integration of bitcoin payment gateway in your websites?
Depends on the payment gateway you are going to use.
Most of them charge between 0.5 and 1% per transaction. You may want to take a look at their respective websites to know more about their pricing.
If you run a small business then you may consider choosing one of the gateways which provide their service for free for the first X transactions/$.
I wouldn't recommend BitPay though.