This is the cheapest one on that I would use:
Cooler Master GX 450W Rs. 5,000.00http://www.czone.com.pk/power-supply-cooler-master-power-supplies-cooler-master-gx-450w-power-supply-pakistan-p.559.aspx
However it is not really suitable for a system with a 6790 because the 6790 needs two 6pin PCIe connectors, while the GX 450W only has one.
The 550W version does have two PCIe connectors.