My experience is that an average 2 bedroom appt is around QR12k per month. You can get cheaper but the standard is quite low (furniture and fittings old and drab, insects). Villas around around QR13-15k per month.(Villas can be much more expensive depending on location, size and amenities) You may need to pay for your own utilities (water and electricity) around QR800-1000 per month. You generally need to pay for your own internet.
You usually have to pay one month deposit, one month rent in advance and if you are using a real estate agent you need to pay them half a month rent as commission. As an example, if you are looking at a 2 bed 12K appt you need to have QR30k available before you move in. You do not want your checks to bounce.
Places like the Pearl are much more expensive.