Yes if you follow an ordinary muffin recipe (I detest the word cupcake - it's so american
) but replace the sugar with splenda or somthing similar you shoud be okay. Actually if you have a look at the splenda website you might find something suitable.
Edited to say that I've done a rough calculation based on the recipe i use and I can get it work out at about 6 syns a muffin but this would be more depending on what you add flavour wise. Let me know if you want the recipe and I'll post it tonight when I get home.