If you can't do any of the work yourself (but you probably could...is there a co-op near you?) then it most likely can not be done for less than $800

Mixte frameset: $75
Shimano Nexus 8 wheelset: $300
Tires/tubes: $90
Handlebars: 15-35 (used)
Brakes: $15-45 (used)
Crankset: $15-45 (used)
BB: $20
cable kit: $35
tape: $15
stem: $5-25 (used)
seatpost: $5-25 (used)
saddle $25-250
chain: $15-25
fenders: $35-125
headset: $20 (tange levin)
brake levers: $5-25 (used-new)

That's a range of $690-1155 and does not include any labor. Note that you could score a decent mixte and get the frameset/headset/BB/crankset/brakes and save another $70-130...maybe even stem/post/bars for a little more. It really depends on how much you would like to personalize it...

My 2c.