include file and variable in post… help appreciated!?

I want to include a php file in all my post pages, and then pull a variable from that file which displays a product price. The idea is I can quickly update this in one file without updating each post. I am trying to use the exec-php plugin to do make the php work in the post. (https://bluesome.net/post/2005/08/18/50/)

If this is completely crazy then let me know! I’m a dumb designer, and not sure the best way to do it.

My idea is
header.php code:

<?php include('prices.php'); ?>

prices.php code:

<?php
$price1 = '<p class="price"><span class="sale">&pound;41.00</span>&pound;35.00</p>';
?>

code in the post:
<?php echo $price1;?>

of course this isn’t working ??
ANY help appreciated.. spend half a day with this.

thanks!!