When it comes to (expensive) fun stuff, I try to buy at 30 percent off or more. At 50+ percent, I'm really happy because that's wholesale. Same with major home purchases like appliances ... at least when I shop in advance, not when I have to replace something that's totally broken. If I get 30+ percent, I'm happy regardless of whether it shows up a little cheaper since I could have easily paid a lot more.
For home stuff that's difficult to really cross-shop based on the crazy number of different brands, styles, materials, etc., I buy what I like, and fits in the budget, with a good warranty, without going too crazy shopping around. Mostly because I've gone crazy shopping around in the past - it's hard to really compare 50 different hardwood floorings, blinds, similar-style/different-brand furniture, etc. At the end of the day, you just need it, so find what you like, buy it when you need and don't look back. Sounds like you've already wasted more money in time than you'd have saved at Lowes.