I definitely find things cheap like Bimbo bread (Kof K Parve), parve Duncan Hines cake mix and parve Pillsbury frosting, etc. at drug stores.
I also recently purchased leftover Kretchmas candy at Duane Reade for insanely low prices- OU-D Hershey items like Snickers and Reese’s, star-shaped Stouffer cookies coated in chocolate (OU-D), etc. that I will use for shalach manos (in all honesty, many Valentines chocolates are also OU-D and will be very cheap post-chag).
Paperific has good sales, too.
As I said above, though, the only things I typically purchase from davka kosher stores are cheese, meats, deli, etc. I buy Acme lox and herring and other appetizing at the factory Friday mornings for very good prices, I bake my own challah and make desserts from mixes. I also recently got 2 ice cream makers (one for year-round and one for Pesach) that I use to make parve ice cream and sorbets, although I have a weak-spot for Trader Joe’s parve ice cream and Sharon’s Sorbet, which I get on sale and stock up on.
I find that not buying “heimishe brands” saves me a fortune. I love having a copy of the Chodosh guide, and coordinate my purchases of yashan items accordingly.
I also make my own ravioli. Yoni’s and Unger’s are a fortune, and it is fun to make the dough with eggs, olive oil, flour, salt, and I buy Polly-O ricotta on sale and The Cheese Guy pecorino to make the filling. Delicious.
A caveat: There is a brand of cheese sold in Pic-n-Pay, Holon, and other stores in Flatbush called The Good Life.
They look nice and have a hechsher on them, but I looked on their website, and they claim their cheeses are supervised by Rabbi Dovid Katz of Flatbush. They have an article from Rabbi Sholom Klass, zt”l, saying that their cheese is made from all kosher ingredients, but the issue is that their cheese is not gevinat yisrael according to how the Rambam understands it.
OU cheeses actually involve a Jew pouring the rennet in the cheese. Tosafot (Avodah Zarah 35) say that in Narvonne, they ate cheese made by goyim using vegetarian rennet because they felt that the gezera was made by Hazal because goyim then used animal rennet. Nobody uses animal rennet anymore, but those of you who are familiar with my approach to halakha know that I don’t deviate from the simple halakha of the Talmud and Rambam. Since I believe that we don’t have the right to overturn codified halakha decided by a Sanhedrin, and because I believe sof hora’ah Ravina u’Rav Ashi, I cannot possibly eat cheese that would be okay according to Tosafot, who happen to believe that changing social realities are doche Talmudic law, which is why they allow clapping on shabbat, not washing mayim achronim, etc. I am therefore more than happy to spend the extra dollars on cheese that is truly halakhic. The flip side to this is that if a person (incorrectly to my mind) claps on shabbos and doesn’t wash mayim acharonim, there is no reason logically why they couldn’t eat cheese made by Cabot or The Good Life.