It doesn't make it more expensive for your pal, using Jmail. If anything it's cheaper since he/she doesn't need international stamps if you use the Jmail mailbox option. The cost of downloading his/her letters as well as the mailbox get paid by you, not by the inmate.
Of course you could also choose to not use the mailbox function. In that case nothing changes for your penpal since he'll answer the letter you send through Jmail by sending it to your house instead of to Jmail in Los Angeles.