The best amendment for any soil is organic matter, compost and other vegetative waste. When you buy "soil" you get a lot of what you already have, the mineral portion of soil and a little of what is needed, organic matter. There is nothing wrong with sandy soil that organic matter will not help.

Why do you think you need new dirt?

If the beds are sandy, add compost ... organic matter.

What kind of area are you talking about improving? This makes a difference in how you buy compost (bags vs. bulk) and what you pay.

I tend to agree with the previous posts. Compost will help increase water holding capacity. In addition, if you had a way to add finer grained mineral soil - i.e. silt, silty clay, etc. - this would change the porosity and water holding. It's not that easy to do unless you know a source for that kind of soil. And it should be in combination with compost, not by itself. The problem is you can't go to the garden center and ask for a bag of silt, it's not done that way.