You can use MapLegend.Format property to format numbers that are visible in the legend. This property accepts standard .NET format strings. You can find more information here:
Unfortunately the kind of formatting when numbers which less then 1000 are shown as is and numbers which more than 1000 are shown as 1K is not possible using Silverlight format strings only.
We are planning to allow using of the arbitrary strings as map range labels in the MapLegend control in the future releases of the RadMap. You can check availability of this feature using our PITS:
the Telerik team
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