A friend of mine is expecting, and asked on my Facebook wall what a good deal is for diapers. Personally, as soon as Baby E starts fitting into the cloth diapers I have, I will be switching her to those. (If you missed my post about cloth vs. disposable, you can check out the price comparisons in this post.) But, until then, I am using my stockpile of diapers.
When I was making my stockpile of diapers, I was able to get them through Amazon Mom (which gave me free shipping), signing up for Subscribe & Save (saving me 30%) and using coupons from different parent/family magazines (saving me an additional 20%) for around 5 cents a diaper. That price is very unusual, and since they stopped doing the coupons, I have not been able to find diapers that cheap again.
If you're looking for brand-name diapers, you'll be spending a little more than if you're buying generic. It depends on your preferences and the sensitivity of your baby. I found that Baby E got a bad diaper rash from Walgreen diapers, so for me, no matter how cheap they are, they are not a good deal.
If you're buying Huggies, Pampers, or Luv's then generally Amazon has the best prices in pretty much all the sizes. Though if you are pairing coupons with sales at different stores, a lot of times you can get them cheaper. Below is a list of the prices of Huggies, Pampers, and Luv's that you can typically find on Amazon. If you find a sale at a store and have coupons that makes the diapers you are looking for cheaper than these prices, I'd say it's a good deal, so stock up!
Since we're talking about diapers, I may as well mention wipes. I don't like to pay more than 2 cents per wipe, no matter what brand. So if you find them cheaper than that, I'd say it's another stock-up opportunity!