Maryam it will actually depend upon distances and timming.

Check in the classifieds and maybe you'll get a del from the posters for "private transportation".

But in general it could range from 30 up to 60 QAR per trip. (Once again this is just a guess; and it could be more)

Regards!!

PS: once again this is my guess based on services recently taken, but in the end you can work a deal with the driver if both sides are willing to.