I concur -- the best values are in Chinatown; had a nice lunch at MBA on Tuesday that was around $10, which is pretty good these days. Thi Thi, Pho Pasteur Saigon, the board faves all apply here.
On Stephen Avenue, of the various pubs and casual places, the lowest price is Henry VIII across the street from the Bay, but the food isn't anything spectacular. The new Original Joe's (just past Centre, in a great space that used to be Fantino) isn't much more expensive, and I prefer the food. (I also prefer it to Milestone's.)
Elsewhere, I rely on Smokee Lee's, the Montreal smoked meat shack at 6th Ave & 4th St. Lebanese stuff; I'm currently swinging the way of Falafel King, but I waver back to A&A at times. Sunterra is actually pretty reliable for quality/value --a nice sandwich isn't much more than $5. (The best deal is the Combo Sandwich, which I think is only available at Banker's Hall; it's a three-slice monster with all kinds of goodies, including ham and turkey, but the same price as the others.)
Depending on your definition of eats, there's a number of decent food court places these days, too; Thai Express in the mall, as well as Koryo at 5th Ave Place and in Gulf Canada are both reliable and not preposterous. Speaking of Korean, Matsarang (which John turned me on to) is excellent value for money.
With the exceptions of Henry's and OJoes, I believe the rest of the places I mentioned will give you change back from a $10.
lunch or dinner?
In either case, Chinatown is an obvious suggestion- many threads on here about Calgary's Chinatown, which has good to great Cantonese, Taiwanese, Vietnamese, and, now, Thai... in the core, one place that doesn't get enough mention here is Murph's (conveniently near the Uptown/Globe cinemas and open 7 days too) for excellent pub grub. The Orchid Room in Bankers Hall has upscale Vietnamese (but still not too pricey for DT Calgary) and is open later for dinner on Friday and Saturdays. Caffe Artigiano has superb, superb sandwiches but is more for lunch (closes at 6 or 7 dep on day) but is open 7 days, as is another fantastic coffeehouse with serious food, Deville in Art Central.
I have to mention that even though it's a chain, Milestones on Stephen Ave side of the Marriott is a nice space with decent food at reasonable prices and very good service.
Vietnamese joints in Chinatown are solid. I prefer Sunny, upstairs on 2nd Ave just east of Centre. U&Me on Centre had tasty dim sum (though I am absolutely no expert). 3 of us couldn't eat everything we ordered and the tab was just over $30 for all of us. Thi Thi Vietnamese Subs on 1st St SE across from the Harry Hays makes the best sandwich downtown at any price (and they'll set you back about $6).
That's my take... I'm sure others will come to your rescue soon enough!