It really does depends on how you define inexpensive, as I've seen a lot of beers from big name breweries (BBC, SN, Stone, etc.) jump in price over the past couple of years to where most 4/6pks of the highly rated porters/stouts are around $9-10 bucks.
$9 or $10 for a 4pk is inexpensive to me since I only buy one or two a month at the very most... and even then that is rare.
But I would probably say:
FBS $10.49/4 (has to be my favorite, would jump at this price any time because of how good FBS is)
Brooklyn BCS or North Coast Old Rasputin, each @ $9.99/4 (good value for quality and ABV too)
For even more value:
Sam Adams Cream Stout - $8.99/6 (when I'm headed to a party or bbq)
Pilsner Urquell 4pk 16oz cans - $4.99 (when I want something light to pair with grilling, watching the game)
Usual suspects for inexpensive beers:
Sierra Nevada - particularly the Torpedo but especially the Pale Ale in the '80s and '90s; still like it, still buy it and it is the go to beer as it's widely available. Find it at every wedding reception. It holds better in bottles than Stone so Sierra Nevada is the one in my fridge.
Anchor Steam - what got me through until Sierra Nevada came about though Anchor was damn difficult to find; still like it and still buy it when I can find it (on tap at Barney's in Westwood; Go Bruins). Tough to find in bottles buy always buy some for the fridge when I do.
Pilsner Urquell - one of the few lagers I can stomach (along with the steams). Not counting the steams, the ONLY lager I'll drink are the pilsners. Try to keep some of these on-hand for guests who were dropped on their heads as children.
Chimay - too sweet for me now but I knew I was making a decent living when I could drink this any damn time I felt like it. Now I just load up my luggage when I visit Westvleteren, and I can spell Westvleteren without looking it up in my beer dictionary
Will still buy some on occasion and keep in the fridge. Looks damn good in those Chimay glasses, and people love the Chimay bottles.
San Miguel - in the late '70s when I was feeling flush at 21 and 22 years of age. Thanks to youcantmakeme for the memory prompt, and I think I'll grab some after football season as I only drink ale from growlers from Alpine and Ballast Point during football. Use to be in my fridge and I think it will make a small comeback.
Dos Equis - go to years and years ago when San Miguel wasn't available. Good memories, and still buy it occasionally but my tastes have changed considerably.
Inexpensive beers I use to keep in the refrigerator, and that almost made me swear off beer:
Heineken (relative to prices nowadays)
Anything Rainier, Olympia, etc.
Anything Coors, the crappiest beer to ever grace these shores
I don't drink a lot of beer but it's going to be a damn good beer even if I'm having one glass. Life is too damn short to drink crappy beer. And it's still damn cheap when you compare it to good wine. I just refuse to drink sh!tty beer. What's the point?