This is a name of an actual product according to wikipedia and of course homer's beer of choice, i think it is worth mid $xxx - high $xxx for resellers.
You may be onto something with this one, duff beer is actually brewed in "real life" - Only seen it for sale in one bar in Glasgow though. Extract from: http://icscotland.icnetwork.co.uk/eatdrink/pubsandbars/guide/glasgow/tm_objectid=12154959&method=full&siteid=50141&headline=firewater-name_page.html "Firewater bridges the yawning chasm between "proper" bars, and the student union. You can drink bottles of Duff beer like Homer Simpson - a regular student hero." "Open: Mon-Sat, noon-midnight; Sunday, 1pm-midnight. Food served: Mon-Wed, noon-3pm; 12pm-6pm, Thur-Sat. Drinks: Pint, £2.20; bottles, £2.30 (Duff £1, Mon, Tue)"
Just googled it it some more, apparently it is actually Coors beer but they have rebranded it with the "Duff Beer" label - No idea how the hell Coors agreed to that one (assuming they know!) Shooosh - Don't tell them! Firewater Bar in Glasgow seems to be only place on the planet that sells it.....