Assign Label To A Text Value Returned From A C# Method?

I have several labels in my application, and I need to manipulate every single one of them. So, I thought the easiest way would be to simply call the manipulating C# method from the aspx page when needed, but oh, I was wrong. I have spent a couple of hours now trying to figure out how to do this, but I have not come to a solution ..

Aspx page:

<asp:Label runat="server" Text='<%= ManipulateLabel("Information") %>'></asp:Label>

Code behind (C#):

protected string ManipulateLabel(string label)
{
return label+" some value";
}

I do not wish to assign each label to value directly from code behind (normally I would do it like this but this time there are so many labels that need a manipulated value that it simply would be too hard updating the application in the future).

When using the code behind, the application simply prints "<%= ManipulateLabel("Information") %>" on the page, which is not what I want :( Also, if I use:

<span><%= ManipulateLabel("Information") %></span>

, it suddenly works. But, I need to use the asp.net web controls, not normal HTML tags.