When you are buying panels in a large quantity(12kw), it is possible to get a much lower price than the listed retail. Probably lower than you would think.
I would not recomend buying a few panels here and a few panels there. You may save money; you may not. What will happen, however, is that you will end up with a system of mismatched panels that you wont be able to get the specified power out of.
For instance: if you have three panels in parallel
a 17v panel, a 23v panel, and a 26v panel = will add up to 17v
and if you have three panels in series
8A panel, a 9A panel and a 7.5A panel = will add up to 7.5A
The panel will the smallest current and the smallest voltage will control your power output. You will probably have to have 15kw in missmatch panels to power a 12kw system. The best way to ensure this doesnt happen is to get panels of all the same brand and model.
Panel prices are at an all time low thanks to the summer 09 international market; and there is a 30% federal tax incentive in the United States; so you have picked a good time to go shopping. I would recommend getting an inverter and the maximum amount of panels for that inverter. If you want to add panels at a later time I would add an additional inverter with its maximum number of panels.
Give Aurora Power & Design a call at toll free 877-337-2490 and we would be glad to give you an estimate.
Hope that helps!