When you're on a family plan the first two lines are included in the price and then lines 3-5 are 9.99 a peice if you are on just a talk or talk+message plan. If you are on the everything data family plans then you're looking at 19.99 for lines 3-5.
The lowest individual plan is 29.99 and that just the basic 200 min plan that has nothing extra included.
He is probably going to switch it to a family plan with a base cost that is only $10 more than your individual plan.
There are a few possibilities here:
1) You have an old plan that is more expensive than current individual plan offerings
2) He sees you are on a plan that has too many minutes, and is planning to switch you to a plan with less minutes (still maybe enough to cover your usage) but two lines, and $10 more expensive, and if you cancel during the contract 2 early termination fees
3) He is only looking at the base cost of the plans and not considering or caring what your actual usage is, not considering if you would go over the plan.
In any of these scenarios he is out for commission so don't do it... I...
If you know you are not going to cancel in the 2 yrs and you are looking for a way to sort of 'finance' the phone, this actually isn't always really a 'bad' deal, but it's not a 'good' deal either.
Chances are he's not doing you any favors here. You are probably better off buying the phone outright or with whatever discount you currently are eligible for, and selling your current phone... than paying the extra $13ish a month (there are flat per line fees in addition to the $10)