There are fees at every step, but the main fees is the deposit fee if you use your credit card, depending on your bank it should be something between 2 to 5% with a minimum I think, all the informations are on the coinbase website
I am aware of the coinbase's credit card deposit fee which is 3.99 %. But I am wondering about the creditcard fee. If I use my credit card to buy stuff, the credit card company doesn't charge anything, but if I want to withdraw money from credit card, then it charges a high-interest fee. So my question is if I use my credit card to deposit money in caoinbase do I have to pay the credit card withdraw money fee?
many thanks

Depends on your credit card company ToS.
I have one credit card that refuses purchases at Coinbase.com ,while my two other cards work completely fine.
However I'm sure
most credit card companies will consider this type of purchase as "standard digital goods" purchase, hence no money withdrawal fee.
oh, that what I want to hear. ...because I don't mind coinbase charges 4% because I probably will make that money there. but if credit card company also charge it probably not worth it.