Getting a span element fill the space in a div

for this you need to change the html structure like this

html

<div class="container">
  <span class="left">Title</span>
  <span class="right">value</span>
  <span class="center">&nbsp;</span>
</div>

and here is the css for .center span

.center {
    text-align: center;
    border-bottom: 1px dotted blue;
    display:block;
}

jsFiddle File

If you reorder your HTML, you can get a simple solution:

<div class="container">
  <span class="left">Title</span>
  <span class="right">value</span>
  <span class="center">&nbsp;</span>
</div>

Place the two floated elements ahead of the .center element. The .center element will be in regular content flow and wrap around the left and right content.

The CSS:

.center {
    display: block;
    border-bottom: 1px dotted blue;
    overflow: auto;
    position: relative;
    top: -4px;
}

.right {
    float: right;
    margin-left: 10px;
}

.left {
    float: left;
    margin-right: 10px;
}

.container {
    width: 200px;
    border: 1px dotted red;
    padding: 5px;
}

When you float an element, the display type computes to block, so no need to declare it.

Also, for .center, if you add overflow: auto, you constrain the block so it does not extend beyond the edges of the floated elements. As a result, your bottom border does not underline the title and value text.

Finally, you can add position: relative and move the .center up a few pixels to align the border closer to the baseline of the text.

Fiddle: http://jsfiddle.net/audetwebdesign/DPFYD/