I send BTCs from x own addresses to y partner addresses and get back the change on 1 more own address I will have to pay for
x * 149 + y * 34 + 1 * 149 +-10 = .... vBytes
Correct?
First of all note that the transaction fee depends on number of inputs not number of addresses you spend from.
Assuming you have received 5 transactions in a single address and now you want to spend all the fund, your transaction have 5 input.
Seceonly, when it comes to calculating the transaction size, there is no difference between the change address and the recipient's address.