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>

Declined with the following reason - chart series are preserved in the ViewState by default. You can do one of the following:
1) Either disable the ViewState of the chart or its container:
ASPX:
<asp:Panel ID="Panel1" runat="server" EnableViewState="false" />
C#:
		RadHtmlChart chart1 = new RadHtmlChart();
		chart1.EnableViewState = false;
2) Or clear the chart Series collection after adding the chart to the page:
		RadHtmlChart chart1 = new RadHtmlChart();
		chart1.ID = "myChart";
		//Add the chart to the page
		Panel1.Controls.Add(chart1);
		//Clear the series after adding the chart to the page
		chart1.PlotArea.Series.Clear();

For the time being the following workaround can be used:

    <style>
        .k-chart .k-tooltip, .k-sparkline .k-tooltip, .k-stockchart .k-tooltip {
            background-image: url("//:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABQAAAAWCAYAAADAQbwGAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAADNJREFUeNpi/P//vwMDFQELEP8beQb+HTWQYgP/DHoD/466cAR4edRAyg38P6hLbIAAAwCnWhhVsxvdCAAAAABJRU5ErkJggg==") !important;
        }
    </style>

Currently labels of SeriesItems with values greater or equal  to the YAxis MaxValue are being cut:

    When chart title is not set.
    AND the labels position is set to Above for AreaSeries, LineSeries, ScatterSeries and ScatterLineSeries.
    OR the labels position is set to outsideEnd for ColumnSeries.
    In FireFox, Opera and WebKit browsers.

For the time being the chart title can be set with space character.

Currently the datasource columns which have space in their name cannot be passed to a databound RadHtmlChart. Add the ability to handle the spaces in the names of these columns.