Sending a letter through certified mail costs at around $6.50 per certified mail piece.
Some places may be a bit higher and some may be a bit lower but whenever I had to send a certified letter from my local post office it cost me $6.50
There's no guarantee that you'll get the letter to the recipient though even through certified mail.
But if someone does get the letter the addressee must be the one that signs for the certified letter to get it to them.
If no one signs for the certified letter then it will be sent back to you at no additional charge.