So - what I am seeing here:
A) Conversion of existing bike with all the parts - $50-$250 with the cost jump coming primarily from buying new wheels v. repurposing old
B) Fitting all components on a Frame/Fork - $500-$600 for the wheels and basic components. More if you are a big enough idiot to go with a French bike.
C) Custom conversion - $650 and up - new paint, full components, good wheels

Thanks to RoyIII for making me feel frugal in my build...