If negative numbers are required to be displayed in brackets , e.g. ($1234,56) instead of -$1234,56, you should apply the desired NumberFormatInfo.CurrencyNegativePattern http://msdn.microsoft.com/en-us/library/system.globalization.numberformatinfo.currencynegativepattern(v=vs.110).aspx
However, the NumericCharacterTextBoxProvider clears the brackets.

CultureInfo culture = CultureInfo.CreateSpecificCulture("en-US");
NumberFormatInfo info = new NumberFormatInfo();
info.CurrencyNegativePattern = 0; 
info.CurrencySymbol = "$";
culture.NumberFormat = info;        
this.radMaskedEditBox2.Culture = culture;
this.radMaskedEditBox2.MaskType = Telerik.WinControls.UI.MaskType.Numeric;
this.radMaskedEditBox2.Mask = "c2";            
this.radMaskedEditBox2.Value = -1234.56;