Hair color isn't always about just "color". It's about shades, hues, and a good cut that will show off your new color. Perfect color doesn't come in a bottle. If you are hoping to get a color correction for past mistakes (caused either by you or another salon), it will take a real expert to add in the right toners to counter the faulty shades you are trying to get rid of.
Almost as important as the color choice is deciding how you will get there. Is this dye job a DIY project or will you put your tresses in the experienced hands of your colorist at the salon? The price tag can be considerably different depending on which direction you go.