Cheap way of low side current sense (under 0.5USD)

[Buriedcode]

I can make a -5V with MC34063 that can supply 50mA with 0805 inductors and being cheap, but I prefer to use a single supply design tough!

You can't use an 0805 inductor with the MC34063.  Well, ok, you can, but it either won't work or be horribly inefficient. Even for very low current applications the MC34063's design requires a minimum load, and its switching frequency  means a rather large inductor.  0805 inductors are generally for filtering, not power applications, unless the inductance is very small (<10uH) and its low power (<100mA).

As many have suggested, a charge pump is the way to go if its just for the negative rail of an opamp for ground sensing. Current will be what, 5mA? maybe 8mA max. That lends itself to either a dedicated IC charge pump, or just a discrete one with two diodes and two caps running off a square wave (555, 74xx ttl oscillator, discrete astable multivibrator, you choose).  As you probably won't need more than -2.5V you won't even need to use schottky diodes, good 'ol 1n4148 or 1n914 will do.

But a dedicated charge pump IC (as recommended several times, by several people) is generally the easiest - requires two caps and you're done.

[ali_asadzadeh]

Thanks guys, The problem is not how I generate -5V, Ok I will choose 7606

The problem is with in the low price of current sense! what's your idea about adding a RC low pass filter after the shunt, before the Op-amp? something like this

[cellularmitosis]

Did someone say ICL7660?

https://www.eevblog.com/forum/projects/an-evening-with-the-icl7660/