Your best bet is to contact a professional installer in your area and get a free quote. There are several factors that make it hard for someone on Y!A to give a general answer that will work for your particular location.
It depends on:
1) Your local electric rate structure. Does the electricity change price after a certain number of kilowatts baseline is used? Will the power company pay you back 100% for electricity you generate?
2) Your latitude and local climate. Is it foggy or sunny? Is this way up north, where the sun is low in the sky? Are you at the top of a mounain, where the air is thin? What are the temperatures like? What is the wind like (airflow cools the photovoltaic panels, making them more efficient).
3) Site dependent details. Is there a south-facing roof? Is the only choice east- or west-facing? What is the slope of the roof? Or will you do a ground-mounted system? Are you in the middle of a forest with tall trees all around, or in big-sky country, with a 360-degree view of the horizon? Does a neighbor's chimney cast a shadow on your roof anywhere?
4) How much electricity does your house use, and how much photovoltaic do you want to install? The cost is not linear per watt. Installing 10 kW takes almost the same fixed labor as 2 kW, so overall costs are lower.
A local installer will know these things by visiting your house and looking at your electric bills.
If you need a rough guide, I think you can see $3 / watt net in the coming year, depending on location. That's what my neighbor got, but it was a deal ahead of its time. Our system cost $4 / watt net.
See the link below for the trend in solar residential pricing.