Hi Bry Bry, log into self serve and check your negative tab balance (how much you're owing on your tab) you would need to pay that off before upgrading. Then you can choose either Tab S or M on the 5s.
Tab S - 150 off
Tab M - 300 off but requires at least a $30 plan and there is a monthly $5 tab charge.
So in order to calculate the costs it would be, negative tab + 275 (if you choose tab M). I suggest Tab M since you get an extra 100 off the phone at the moment. Hope that helps. :)