The local currency is the Brunei Dollar B$ you might hear Ringgit used to refer to the Dollar but be sure that participants are not talking about the Malaysian Ringgit MYR which is valued at less than half a B$ .
As of January 2010 $1 BND = 2.421 MYR = 0.449 GBP = 0.512 EUR = 0.705 USD
The Brunei Dollar is tied to the Singapore Dollar at a 1:1 rate. By law both currencies can be used interchangeably, so if you're coming in from Singapore, there's no reason to change money as your cash will be readily accepted. Likewise, any leftover Brunei Dollars can be used at par in Singapore. However, many stores refuse Singapore notes with seemingly microscopic tears in them, and notices to this effect are posted at the cash register. Malaysian Ringgit RM will also be accepted in a pinch, but the exchange rate may not be in your favour. The Ringgit is not available at Brunei banks but can be obtained from moneychangers.
The Brunei Dollar is divided into 100 cents. There are banknotes (http://www.finance.gov.bn...) from B$1 to a whopping B$10,000 handy if you're shopping for Rolls-Royces and coins (http://www.finance.gov.bn...) of 1 to 50 cents. All smaller notes and the 2004 series of larger notes are printed as brightly coloured polymer notes.
costs
By South-East Asian standards Brunei is roughly on par with Singapore, meaning roughly twice as expensive as neighbouring Malaysia. You can reduce costs by eating at local restaurants and avoiding the more expensive restaurants in hotels. Budget accommodation, once very limited, has expanded in recent years and you can now get a decent bed for the night for around B$30.