If you consider the 2 circuit used to craft the solar pannel, it needs 10 Refined Iron to craft.
But the cost is high too in Redstone (2 for the battery, 2*2 for the circuits) if you consider there's strangely less redstone with IC² (only my, maybe, but it happen me with more than 20 worlds) than without.
And last but not least (If you don't have tons of rubber Tree, for example at the begining), it cost you 13 Rubber ...
I don't think it's a good Idea to decrease the output of solar panel, when Windmill are easier to get, produce more EU, with or without sun.
Solar panel are just easier to use, considering that a 512 EU/t solar station is easier (but not cheaper) to build, even if there are means to make the same with windmill.
And for 0.1 EU ... it's ... just 30 solar panel for 1 windmill (assuming you use it very high and in good biomes, and of course because it works day AND night) ?
I don't speak about Nuclear reactor, of course ...
Maybe you understood it, but I love using Solar Panel: in big farm to produce UU matter