I agree that they are expensive but not out of line.
ARB $300 20% 17 AMP
Extreme Air $400 100% 35 AMP
Currie Air $490 100% 32 AMP
So it is about $100 to $150 more for a 100% DC compressor.
Not that crazy if this thing is going to last years.
The dual batteries should take care of the extra AMP draw.