Put prickly pears in a bowl or dishpan full of water. Holding each one with tongs, scrub with a vegetable brush to remove dust and some of the stickers (the rest will come out later when you strain the juice).
Transfer 5 at a time to a blender jar and process until liquid. Line a mesh strainer with cheesecloth and strain juice into a medium saucepan. You should have about 1 cup.
Add lemon juice and sugar, and slowly bring to a simmer. Cook until syrup begins to thicken. If you want a thicker syrup, stir in 1 teaspoon of cornstarch dissolved in a little cold water and cook to thicken.
Stir with wire whisk if necessary to smooth consistency.
For diabetics: Use an amount of your favorite non-nutritive sweetener equivalent to 1 1/2 cups of sugar. Since sugar causes thickening, you'll have to compensate by using a little more cornstarch, or use another thickener such as guar gum or xanthan gum.