The Saudi currency is the Saudi riyal (ريال, SAR), which has traded at a fixed rate of 3.75 riyals to the US dollar since 1986. The riyal is divided into 100 halalas, which are used to mark some prices, but in practice all payments are rounded to the nearest riyal and chances are you will never see halala coins. Notes come in denominations of 1, 5, 10, 20, 50, 100, 200 and 500 riyals, with two different series in circulation.
The riyal is also pegged to the Bahraini dinar at a ratio of 10:1. If you are considering travelling to Bahrain, virtually all shops in Bahrain accept the riyal, but the dinar is not as easily convertible in Saudi Arabia.
Saudi Arabia is still largely a cash society, and credit card acceptance is surprisingly low outside luxury hotels and shopping malls. ATMs are ubiquitous, although the ATMs of many smaller banks do not accept foreign cards; Samba, SABB and ANB are probably your best bet. Money changers are available in the souks, but are otherwise rare. Foreign currencies are generally not accepted by traders.
Costs in Saudi Arabia
Prices tend to be quite expensive: expect USD50/100/200 for budget, mid-range and splurge-level daily travel costs.
Tipping is generally not expected, although service staff are always happy to accept it and taxi fares are often rounded up (or, not infrequently, rounded down). In expensive restaurants, a 10% service charge is often added, although due to lax regulation, many employers simply usurp it (ask your waiters whether or not they get any of it if you want to tip them). There is no sales tax in Saudi Arabia, and no income tax either!
What you should buy in Saudi Arabia
Only a few local products are of interest to tourists. Locally grown dates are of high quality, and religious utensils are widely available but almost exclusively imported. Copies of the Koran are produced in a variety of editions and sold at very low prices. Zam Zam water is available throughout the western region and at all airports.
Carpets are a popular purchase, most of them coming from nearby Iran. Especially in Jeddah, there are many carpets, many of them brought by pilgrims who sell them there to finance their journey to Mecca.
Large gold and jewellery markets can be found in all major cities. Haggling is the norm in most small to medium sized shops. Makkah and Madinah offer a wide variety of luggage, clothing, jewellery, knick-knacks, souvenirs, toys, food, perfume, incense and religious literature, audio and paraphernalia.
Large, well-maintained, air-conditioned shopping centres and grocery shops (e.g. Safeway, Geant, Carrefour) are scattered throughout the Kingdom.