You can spend $6 for a gallon of paint or $60. So how do you decide when to pay more or less? There are a number of important aspects to examine:
2) Performance requirements
4) Color (yes, this makes a difference)
5) Labor savings
The first is self-explanatory; sometimes you don't have the money to pay more, so it simply isn't an option. If you do have the means, however, then it's time to take a look at the other factors.
How long do you want the paint job to last? If you want your new coating to look good and perform generally well, then you will need to move into the middle-of-the-road option or higher. The very cheapest paints have little or no titanium dioxide and won't last long, even on the interior of your home. The most expensive paints have the best color retention (interior and exterior), washability, adhesion, and final appearance.
Where is the paint going? While you get what you pay for with both interior and exterior paints, it is better to put the extra money on the outside of your home. Here in Colorado, our harsh sun and temperature extremes, along with the winter snow are extremely hard on any coating. If you can afford it, get the best. If you're not so picky, you can save a bit on interior paint, especially in low-use areas.
Deep colors don't cover as well, so if you plan on putting red in your kitchen or yellow in the kid's room, you may want to think about springing for Benjamin Moore's Waterborne products. Prices range from fairly to very high, but you'll need fewer coats. Combine the savings in material cost with labor, and you may very well come out ahead.
There are other products which can save you a lot in labor. They are diverse and too many to cover here, but a good paint contractor can help you with selection.
As always, you can always contact me if you have any questions!