What do I need in order to be able to send a pet in a flight overseas and how much would it cost?

I'm thinking about possibly sending my 4 kittens to my boyfriend in germany. I've tried finding them homes here but it seems no one is actually going to take them. So what do I need for them to be able to travel overseas and how much could that cost? Hes willing to pay their flights but I'm thinking for each of them it might come to around 1200 dollars? Or more considering its overseas.