How expensive to replace the slide out awning fabric?

Absolutely best replacement topper material is available from Stone Vos in Brooksville, FL.

Many folks also recommend Tough Tops for slide topper replacement.

Regardless of where you get the material, it is not hard to R & R. As noted, you need a couple of good ladders and a second person if the toppers are long. There are a number of You Tube videos showing the process.