Sancerre is a well known source for quality sauvignon blanc. There are exceptional soils in the region and excellent wines if you know where to look. The fact that it's well known also leads to a bit of "branding" which in turn leads to some less than thrilling, overpriced wine. Let me explain. Since Sancerre is somewhat famous, consumers will seek out wines labelled Sancerre and most are willing to spend a decent amount for these bottles. This built in market can make it easy for less caring companies and producers to make wines that are decent and acceptable, but lack distinction or expression. They can still sell all of it and there's no incentive to do anything in the vineyards to make a better wine. Wineries can over produce the amount of grapes coming from a vineyard, cut costs by machine harvesting, use less strenuous methods for the sorting and selecting of grapes, and formulaic methods of production like laboratory selected yeast strains that finish fermentations quickly and give prescribed flavors.
My issue isn't necessarily with these high production wines per se, it's with the fact that in this case, a lot of these wines cost over $20 a bottle just because they are coming from a well regarded region. Give me the same wine at $13.99 and I don't have a problem. Also, this not to say that good Sancerre isn't worth it, because it is. As I mentioned, the chalky limestone soils in the area can make excellent wines and if you find one of those, I'm talking about wines with balance, character, mouthfeel, and a unique sense of mineral, the higher price tag is totally justified.
The good news is that Sancerre is just one area of a large region in the Loire Valley that produces sauvignon blanc. The areas near Sancerre make some really nice everyday wines and also have some talented, energetic winemakers striving to make something exceptional. Because these neighboring areas aren't as well known it goes to form that the wines from these places don't cost as much and they are a great place to look for good value in sauvignon blanc.
Good basic alternatives around $15 are wines from near the city of Tours, labelled Touraine Blanc or Val De Loire. Further east, wines from Pouilly-Fumé and Menetou-Salon will have a heftier price tag but can rival the quality of good Sancerre. Producers in Reuilly, Quincy, and the Coteaux de Giennois also make wines worth trying. Another wine to mention is from Saint-Bris, near Chablis and actually part of Burgundy, but designated for sauvignon blanc because of the soils and the geographic proximity to the upper Loire. Finally, if you want to leave the Loire try a white Bordeaux, it's usually sauvignon blanc blended with sémillion and sometimes muscadelle and there are some real deals to be had.
I hope this helps you find some new wines!