TCGplayer mid is consistently pretty accurate in showing how much I can buy cards for. It takes the data from a lot of different online sellers, and the mid is the average of those costs. I prefer it to the TCGPlayer Low, because the low price dosen't factor in shipping, and once that's all and done, the mid is pretty close to what I'd pay anyway. Some of my LGS even pull up TCGplayer and grab prices right there when selling singles.
That said, perhaps if there are other price guides with APIs those could be added as options as well?