Window portraits are great and the setup is cheap as others have said. But what makes them visually great?

1- A large light source. This gives a soft effect on the subject. The equivalent to this in studio lighting is a big soft box. The larger the source and the closer it is to your subject the softer the look. A beauty dish is nice too but considerably smaller so the effect is harder. As you get down to 6-10" reflectors with snoots or honeycombs you get into Hurrel's territory.

2- Direction. When shooting window portraits you are working roughly perpendicular to the light path. Having direction in the lighting, a difference right to left, makes more believable/interesting portraits IMO. You can use a reflector on the off side to fill/control this effect.

Ok, so next.

If you want your background pure white think over-exposure even if the back ground is white. With an incident flash meter at the background I'd set the lighting to be 2 stops brighter than the reading at the subject. This makes a perfect white with no detail. The opposite if you use a black background and want black.

With the white background 2 over you need good separation between subject and background. Controlling this distance will control the aura. Too close and the subject gets fuzzy around the edges. To find where too close starts, meter behind the subject's head pointing at the background, that reading should be equal to the measurement at the subjects nose pointing at the camera.