For the cost of living, few years back this could have been a valid point if you are trying to compare the three main cities in Saudi Arabia Jeddah, Riyadh and Dammam ( when we say Dammam we usually mean the three attached cities of Dammam, Khober and Dharan). Accommodation ( not in a compound)being the main comparing factor , they are more or less are all the same now.
For other items like Electricity, Tel, food…. The prices are fixed for all.
Not sure about School fees comparison between the two.
All on all, no real differences on the cost of living, it’s not like the difference between London and Cardiff for example.
You will not find many South Americans living in Saudi Arabia, but for sure you can find more Spanish speakers in Riyadh in comparison to Dammam due to the number of embassies and companies headquarters.I personally know only two Brazilians and one Venezuelan In Khobar