I cant confirm that this deal is for every walmart or just the one near me (Lafayette, LA) but it even beats the newegg price which is pretty damn good considering theres no shiping cost.
I've seen places like geeks.com run sales in the past couple of months on 500gb external Seagates for around $100. You can get a 500gb WD external from buy.com right now for $133, shipped free.
I need an external for backup purposes, but personally I'm gonna watch for another sale & snag a 500 for around $100. $90 for half the size (well, $99 for me after 10% tax) doesn't sound that great.