[Ryland]

You did not give enough info.
We need to know the battery size and how much you are taking out of it every day, if you are not using it every day then how often are you and how long can you have it charge for, if you are charging it some other way and only need to do a float charge while they are in storage for 6 months or more then a solar panel that puts out 3-5% of the capacity of the battery should be fine as on average you should get around 4 sun hours per day, so a pair of batteries that gives you 300 amp hours at 12 volts should be ok with a 10-20 watt panel, so $50-$200 for something that will work, any larger and you would need a charge controller as well, to small and it's hardly going to keep up with the self discharge of the batteries.
You question is kind of like saying "I have a car, how much am I going to spend on gas for the next ten years?" when buying solar to charge a battery yes, you need to know the battery size and type, but mostly you need to know how much power you need to produce and that is based off of how much you used.