Are you using Tier 1 panels?
Your costing largely depends on the quality of your panels which is defined by the make and model of the solar panels.
The make and model will give out a lot of information of the manufacturer, the model will give you details about the technology behind your solar panels and how much research has been done to design those panels.
And this brings you to the 3 tiers into which Solar Panels are classified. Target Solar only deals in Tier 1 solar panels. Tier 1 panels are amongst the most reliable panels.