Help

I want to get my boyfriend of 1 year and 4 months a promise ring, we have a 2 1/2 year age difference i being the younger one of 16. He is a marine and going to go to Iraq, but before he goes he gets to come home, around Vlentines day, i want to buy him a promise ring, to let him know that while he is gone i promise to always love him. How much do i spend? and do i have to buy the rings in a set ? am i too young to buy one??
Please help .......