A workaround is to use the ClientTemplate and change the strings manually with some JavaScript (http://www.telerik.com/help/aspnet-ajax/htmlchart-client-templates-for-tooltips-and-labels-execute-javascript-and-display-html.html), e.g.:
<telerik:RadHtmlChart runat="server" ID="RadHtmlChart1" Width="400px" Height="400px">
	<PlotArea>
		<Series>
			<telerik:BarSeries>
			<LabelsAppearance>
			<ClientTemplate>
				#if (value > 0) {# $#=value# #} else {# -$#=value * -1# #} #
			</ClientTemplate>
			</LabelsAppearance>
				<SeriesItems>
					<telerik:CategorySeriesItem Y="20" />
					<telerik:CategorySeriesItem Y="10" />
					<telerik:CategorySeriesItem Y="-30" />
				</SeriesItems>
			</telerik:BarSeries>
		</Series>
	</PlotArea>
</telerik:RadHtmlChart>