My cheapest ever was Molly Millions, as it was essentially a closet cosplay. I just had to thrift a few items and it came out to about $20. http://www.cosplay.com/costume/441711/
My most expensive by far, Quorra from Tron:Legacy. Over two versions I've probably spent $1000. http://www.cosplay.com/costume/475473/
My AVERAGE costume cost? I save where I can, but I live in Canada where fabric prices and sales taxes and shipping rates suck, and I like big flowy yardage-intensive costumes, so most of the time my costumes work out to $200-300. I've managed gijinkas and originals and simple costumes on about $100.
...if you want serious detail I actually keep cost sheets on every single costume I made, all the way back to my first....
...this hobby is expensive.