I haven't decided if I'm doing anything special for "National Margarita Day".
While this could turn out to be far from a "cheap" proposition, Ole (Inman Sq) and Zocalo (Back Bay), is offering free guacamole with every margarita ordered, which appear to be in the $10 to $13 range.
Get one, and it's a nice post-work snack -- after that, you're on your own.
Not into tequila? Why not try to wrangle a free Narragansett Cream Ale...
There's a re-launch party (6pm) for the old-school brew at The Salty Pig across from the Back Bay T station. The Dig has a good write-up about the beer itself. Besides liquid samples, expect to find something to nibble on.
Tweeters also have a chance at a Cream Ale prixe pack.
Check out other cheapness for today and tomorrow.