WalMart paint (ColorPlace?) was closer to $15 a gallon when I did it a few years ago. It was very highly rated in CR, maybe even above Valspar at the time.
They also sell Kilz brand paint that is quite inexpensive and high rated. I would check Consumer Reports - your library probably has a subscription that you can access online through the research link of their website. Buying paint in 5-gallon buckets is cheaper.
Contractor savings could work, but usually those stores already charge more than that percentage higher anyway, for paint that is no better than the ones we are discussing.
In our area, there is a government agency that recycles paint and sells in by the 5-gallon bucket at super low prices. Here is one in your area:
http://www.greensheenpaint.com/index.php Unfortunately the website doesn't list prices.
Not even the best paint really covers in one coat unless it is the same color as what is under it. Even so, 15 gallons seems high to me for an average size house, unless each room is a different color.