The only workaround I can think of would be to use a fake line through. That is where you create a gif with a horizontal line and transparent background. The CSS would be as follows:
HTML Code:
<style>
.linethrough {
background: url(/linethrough.gif) repeat-x center left;
color: #000;
}
</style>
The HTML code would be the same as you are used to.
HTML Code:
<p>Their price: <span class="linethrough">$18.88</span></p>
You would need to create a stylesheet to display a normal linethrough if the document is printed, however, since the background image won't be printed. This method will allow you to control the size, position, thickness and color of the line. If you set the color to be lighter than the text, that should ensure the numbers can be read correctly.