I've found America's Tire to be better than Costco on price (just my anecdotal experience); plus lifetime free rotation/balance.
I prefer America's Tire/ Discount Tire (same place, different names in different parts of the country) over Costco also. Costco carries brands that tend to be more expensive, and that are probably better tires than what I buy, but my experience has been that I don't see the treadwear or other benefit from buying the more expensive brands from Costco (Bridgestone, Michelin). I usually end up with the next-tier down, like Goodyear, General, Continental, etc. I've had mixed luck with Hankook/Kumho. YMMV.
Some tips for getting the best deal possible on tires from America's Tire/ Discount Tire:
1) Sign up for Discount's "Car Care One" credit card. DT frequently runs sales where you get a $60 rebate for using their credit card to purchase the tires. Pay it off immediately, obviously. No annual fee. This is the only store credit card I have.
2) Buy during a DT sale. They do large sales frequently, usually around the time of holidays -- Memorial Day, July 4, Thanksgiving, etc. These sales come in the form of rebates, such as $75 or $100 off a set of four tires. DT processes their rebates quickly, and they come in the form of a debit gift card (e.g. Visa).
3) For those tires you buy on sale, buy ones that are also on a manufacturer promotion.
Important: DT does not let you stack two rebates from DT, but you
can stack a mfr promo with a DT promo. Example: $100 off a set of 4 tires (DT promo), plus $75 General Tire rebate (mfr promo), plus $60 Car Care One credit card promo = $235 in rebates. Look at the rebate forms online before ordering -- you can tell by the address to which you mail the form whether it is a DT promo or mfr promo.
4) Order from discounttiredirect.com, rather than buying at the brick-and-mortar store. Shipping is free, but the per-tire price may be a little higher than the in-store price. YMMV on this one, but if you live in a state where they do not charge sales tax for the web orders, this generally works out in your favor. Example: $125 per tire, 8% sales tax on 4 tires = $42 saved. (If you live in a state w/o sales tax, obviously this doesn't gain you any benefit.)
5) Bring tires to local shop for mounting/balancing. I use Walmart, which will mount/balance/lifetime rotate + rebalance for about $55-60 for a set of 4. YMMV for whatever it costs you locally.
I generally buy replacement tires in advance, during a sale/rebate/promo combination, then store the new set in my shed until I need to mount them.