Fry's Electronics offers the Kingwin 120mm Case Fan, model no. CF-012LB, for $3. This $3 mail-in rebate cuts it to $0. With $2.99 for shipping, that's the lowest total price we could find now by $4, although we saw it for free after rebate a year ago. This 950 rpm fan features an air flow capacity of 40 CFM. Rebate expires March 6.
Update: Note that this item is currently on backorder, but can still be ordered at this price for delivery when available.