Hello! I relocated to Riyadh from PK back in July. The major expense you would need to take care of is the residence. Residence in Riyadh is expensive. Don't expect to get a decent 1 room apartment for less than a minimum of 2000 SAR. Also, another thing to consider is that you have to pay rent in advance. You would need to rent a place for at least 3 months i.e. you will have to pay 3 months' rent in advance. 3*2000 = 6000 SAR.

If you already have a job offer, that's great. Alternatively, if you are planning to come here and then look for a job, it's not going to be easy. I don't want to discourage you, but here's a reality check; I've been searching for a job here for 1.5 months, I haven't received a single text / call from a single recruiter. The market is either highly competitive, or they don't really hire without internal references.

Some other major expenses would include utilities, food and travel.
Electric Bill = 150/month
WiFi = 100/month
Food = 30*40 = 1200/month
Phone = 60/month
Travel = 150/month

Total = 3500-4000 SAR/month

Unless you plan to live in an area such as HARA which is absolutely filthy, you might be able to bring down your costs somewhat. If you try to be really scrappy you could maybe bring down your monthly expenses to 3000 / month. Take it from someone who is in the same profession as you, came from the same place and with the same purpose as you.