If you absolutely do not want to use code-behind, you can try conditional operator for this:
<%# ((int)Eval("Percentage") < 50) ? "0 %" : Eval("Percentage") %>
That is assuming field Percentage contains integer.
Update: Version for VB.NET, just in case, provided by tomasofen:
<%# If(Eval("Status") < 50, "0 %", Eval("Percentage")) %>