Yes, $10 is the mathematical difference. And yes, PREP, PREP, PREP is very important. But, and I say BUT, I called Pratt & Lambert about 1 year ago about their paint at Walmart. And I spoke with one of their paint 'designers'. He said that the paint you buy at WalMart vs. the paint at the Pratt & Lambert store ARE very, very different. He said if the paints were any where near the same, it wouldn't matter where you bought it. Touchette (sp). He said that WalMart's paint is a 'competative' paint with the Retailer's paint. He said if the paints were any where CLOSE to being the same, they'd have a mutiny from their Retailers. They're different as far as solids in the paint go.
Along the same lines, I have friends and neighbors that painted the outsides of their homes, and they are having to paint them again after just 5 to 7 years. Where did they get their paint? Rickels (out of business), Heckingers and Builder's Square (both out of business), Home Cheapo (I hear nothing but complaints about their paint, and my 4 year old daughter is a stock holder), and so on. But as for P & L, Finneran & Haley, MAB, etc., etc., they've been around for years in spite of all the aforementioned Home Centers, in or out of business.
As you can see, I'm a little emotional because I hate when people don't want to pay a pro to do a job, yet, they want to SKIMP on material when they're saving a TON of $$$ on labor. They ought to be taking some of that hard-earned, saved $$$ and spending it on quality material. That's the ultimate in DIY work.
'Nough said. Do have a nice day, and I mean that too.