Walmart typically has the lowest non-sale prices on motor oil, although we have a regional chain here, Meijer, that typically offers similar prices.
The parts chains have oil and filter oil change specials every month that are sometimes a better deal than what you can get at Walmart, particularly if you want to use a high-end oil filter.
Always check for closeout or clearance deals, since these can bring you some really cheap oil.
Also, unless you hate MIRs, there are offers from time to time that will net you free or very cheap oil.
Had you asked this question a couple of years ago, for example, you'd have all of the FAR QSUD, G-Oil and Nextgen you could stand.
There will be more FAR offers in the future and that's one way to build a stash, which will then offer you a selection of oils to use for which you had very little out of pocket outlay.