I live in norway so no walmart etc stores here 
Only 3 large retailers that do overall gadgets in walmart style but not walmart prices or selection sadly.
So its either small fans ment for bathroom exhausts at 15$ or the engine cooline fans i found at 30$.
Next is industrial gear at well not so fun pricing.
on the plus side i do have 12c or less outside 10months of the year, so semi-cold air to use.
downside is that since there is at most 1hot week of summer, there is no demand for diy A/C etc
Only 3 large retailers that do overall gadgets in walmart style but not walmart prices or selection sadly.
So its either small fans ment for bathroom exhausts at 15$ or the engine cooline fans i found at 30$.
Next is industrial gear at well not so fun pricing.
on the plus side i do have 12c or less outside 10months of the year, so semi-cold air to use.
downside is that since there is at most 1hot week of summer, there is no demand for diy A/C etc
You have an auto junkyard near you
Electric engine fans run on 12 volt,get a scrap one or 2.They move pretty good CFM & should be cheap 
They're waterproof,so you could mount them outside & since they're DC,you can reverse the rotation if needed
http://images.biltema.com/PAXToImageService.svc/product/large/2000032589
14" 2200rpm 80watt 12V - 530cfm, so 1060cfm total.
70$ new for both so fairly cheap, hopefully that is enough and if not a third will be added for 1590cfm total.
Rough sketch.
http://i.imgur.com/eNlsyOv.jpg
2x 4" ducts from each fan going to a 2x4" adapters pointing out the window gap (might skip the adapters depending on how flimsy they look)
(live on street level in the middle of a city so not very comfortable leaving the window open beyond its outer locking point.)
got a window on the other side of the shelf also to take in cold air, got room for 8x ducts in the window so can go up to 4x 4" for each fan if needed.
So budget/cost:
Cooling - 110$
shelf - 25$
miners - 3370$
psus/wiering - had already
Total - 3505$, so 0.95$ per gh/s with miners/shipping/cooling etc