Took me all day to figure this out, so i thought i would share the code.
http://pastebin.com/bKrNJeda add this code to your functions.php
Obviously you might want to do a find and replace on the term "unit_price" and give the custom merge tag your own name, depending on application.
Then assuming you wanted to do this: {custom_field:{pizza:7}}
You would write it out like so: {unit_price:pizza_7}
Why would you want to use this? say your user choose a type of product from a drop down list and you want to hit them back with more information about the product they chose. You would create a lookup table of all the possible products as custom_fields on the post this is attached to, and then implement the above code.
Hopefully this helps someone.
-deweydb
