The bigger providers of pre-paid plans allow you to switch back & forth between a monthly data/talk plan (when you are in the states) and a pay as you go plan (when you are outside). This is how I maintain my US mobile number while I am home in SE Asia.
The pay as you go charge you 10 cents per minute for calls and texts and no data, with a minimum charge of $3 per month. But this $3 monthly fee keeps your number / SIM active when you are away from the states.
You can top up your account online in large increments, such as $100 with a one year validity, or in smaller amounts on a more frequent basis.
If you simply pay the $100 top up, but then disable your data plan, your sim and number will expire within six months, even though your payment credit last longer. You need to maintain some sort of calling or data plan on your number for the SIM & number to stay active.
So you really need to ask, is the keeping the SIM & number worth cost (say 9 months @$3). Buying a new SIM is pretty cheap with pre-paid service providers.