Details on PayPal fees: https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=marketing_us/fees
yes, you have discount if you over...... https://cms.paypal.com/cgi-bin/mark...&content_ID=merchant/merchant_fees#id1_header
Paypal has many terms, just read up on the two links above. If that's not enough information browse around on their site and you'll find the answer.
It should be depend on the transaction but they have decided some minimum charge for transaction.There are many people who use the services of online payment also many of them are very popular systems.
I believe the standard transaction fee is around 2.9%, though this can be lowered significantly with different kinds of accounts, and with increased transaction volume.