Importing variables e.g. Price into loads of posts

Sounds like you need to write a custom plugin… In your post, you would type something like [showPrice: 1234]; the plugin would replace this with the current price of item number 1234.

What you wrote won’t work as a plugin, simply because your function has no argument (and because you’re using the wrong hook). Here’s a rough (but tested) alternative draft:

<?php

/*
Plugin Name: Show Price
Plugin URI: http://wordpress.org/support
Description: Displays product price based on product ID.
Version: 0.1
Author: NC(at)WP
Author URI: http://wordpress.org/support

Usage: [showPrice: ID]

*/

function showPrice_install() {
  return true;
}

function showPrice_display($content = '') {
  ob_start();
  $currencySymbol = '&_#36;'; // U.S. dollar; replace if necessary
  $prices = array(
    1 => 5,
    2 => 6,
    3 => 7
  );
  $matches = array();
  $match = preg_match_all('/\[showPrice:(.*)\]/Ui', $content, $matches);
  foreach ($matches[1] as $k => $match) {
    $match = (int) trim($match);
    if (isset($prices[$match])) {
      $price = $currencySymbol . number_format($prices[$match], 2);
    } else {
      $price = 'N/A';
    }
    $content = str_replace($matches[0][$k], $price, $content);
  }
  ob_end_clean();
  return $content;
}

// Plugin core

if (isset($_GET['activate']) && $_GET['activate'] == 'true') {
  add_action('init', 'showPrice_install');
}
add_filter('the_content', 'showPrice_display');

?>

What you can do now is rewrite the population of the $prices array inside the showPrice_display() function, so that the array is populated from a CSV file or a database…

Be sure to remove the underscore from the $currencySymbol assignment; I put it in just to show that it is an HTML entity rather than the actual dollar sign…