How to create your mining rigs cheaper

Hello , although I'm new in mining , I started 2 months ago but I'm familiar with computer hardware , So after a lot of searching on online shops and hardware stores I finally managed to find some RX 480 Graphic cards in a local shop , but you know the price is going higher and higher each day and for someone like me with low budget it wasn't easy enough to buy desired numbers of these cards , so I decided to spend less on other parts like MB and power supply , first I calculated the price of these parts, and then I decided how many GPUs to buy .
for mother boards I chose these :
MSI 760ga-p43 (fx)

details here

msi 870c45 (fx) V2

details here

Gigabyte GA-MA785GT-UD3H

details here

all used no new parts and for power supply i chose :
Green 700w Power Supply for 100$
details here
for CPU, HDD and RAM also I chose minimum means AMD Sempron for CPU 4 GB fury Ram and 160 GB HDD Maxtor
each rig with CPU+ HDD + RAM + MB cost me almost 100$ , i bought 3 new power supplies each cost me 100$ as well also 13 PCI-E 11x to 16x riser cables , I couldn't find USB risers so i had to buy these :

at the end with my own PC at home with a 600w power supply I made 4 rigs 3 GPUs on each but I tested 4 GPUs on one of the rigs with a 700w power supply and it's working with no problem , you can see the LEDs are on in the image below:

I forgot to say riser cables price is between 2 to 4 dollars but not in my country , it's more expensive here but on the other side power price is very low in compare to European countries.