From the airport

Perhaps the best way to get into the city from the airport is by taking taxi. A metered fare to the city should cost around SR
70-90.  Negotiated flat fares are common and are potentially cheaper
than using the meter, but be sure that you  discuss the terms
beforehand. A far safer alternative is to simply plan ahead and book
your hotel’s limousine. These are typically priced as much as a taxi
trip, but the fleets contain mostly high-quality, comfortable cars as
opposed to older, poorly maintained vehicles taxis operate in.

The official taxis use a zone-based flat fare system, with most of central Riyadh in the SR 45-55 zone, however the list of zones is available only in Arabic. The cab drivers are typically knowledgeable with regards to the popular landmarks, though most speak only Arabic, which can be problematic to foreigners.