I just tried a mexican restaurant that I believe just opened in Upper Darby (66th & market), just west of West Philly. It is called Xochimilco (mexico city style), is very good and seemed authentic. It passed the initial litmus tests of good homemade chips and salsa (pico de gallo) and bilingual staff. Actually we were the only people not speaking spanish, which is a good sign in my book, since it is attracting "authentic" customers. Although I did feel a bit offended that the waitress went out of her way to overexplain the menu items to us, but I guess we don't look like "locals". I had tacos huastecos? which were basic steak soft tacos (steak, onion, cilantro and soft corn tortillas with green salsa which was excellent), and my wife had a burro which was steak,beans,white rice topped with mild red sauce and Chihuahua cheese. Overall, everything we had was good (especially the crepas with caramel sauce), and didn't taste out of a can. The menu was small, and prices were good to average (the tacos were $5, but the burro was $10), but the lunch menu looked to be $5-7 for everything.
We are still looking for good cheap sonoran style mexican like we had in Tucson, AZ, but with the crap that passes as "authentic" mexican in this area we were satisfied. I guess after living 50 miles from mexico, everything else is a letdown, but still the only authentic places we've tried were more mexico city style (that moroleon place in kennett sq being the only other authentic place we've been). BTW its a BYOB and has mariachi music via stereo system or live on weekend nights. Buena suerte