I ate at Topolobampo at 7:30 a few Thursday nights ago, and there was virtually no wait to be seated at Frontera Grill. So don't rule it out for fear of a wait that might not exist. (Many of us locals go early to avoid long waits.)
You can also try XOCO, Rick Bayless's new restaurant serving "Mexican street food", right around the corner from Frontera Grill and Topolobampo. Details on
his website.
One strategy to reduce the cost is to go for lunch rather than dinner. All three restaurants are open for lunch during the week, and lunch at Topolobampo is surprisingly similar in price to Frontera's. Also, Frontera Grill is open for brunch on Saturdays.
There's nothing else I would recommend that's walkable (or within "waking distance"

). And I don't think anyplace else is really comparable to what he is doing. We actually have at least a dozen other restaurants throughout Chicagoland serving creative regional Mexican food, most of which are excellent, but none of them are within walking distance (
Salpicon is the closest, and it's a mile away) and none of them are significantly less expensive than Frontera Grill. If you would like to try one of the others (or even just to check out its website), my favorite is
Mundial Cocina Mestiza in Pilsen, near the CTA Pink Line station at 18th Street. Another excellent one that's a bit more unusual and not all that far is
Mexique, which serves French-influenced Mexican food; it's in West Town, two miles west of River North and accessible by the #66 CTA bus that goes down Chicago Avenue. A bit further away are
Fonda del Mar, on Lincoln and accessible by the #11 CTA bus (or a half mile walk from the Irving Park station on the CTA Brown Line), and
Mixteco Grill, a half mile east of the Montrose station on the CTA Brown Line.