Don't underestimate the value of sky replacement. And for $4, an out of country processor I use from time to time created the "day to dusk" image(to the right). From dreary drab to money shot! I charge clients $50 for a day to dusk conversion like this.
Before I saw Rob's example to the right (click on the photo to get a closer look) I would have argued that to get a reasonable twilight exterior shot you need to make a special trip to the property at twilight and charge the client at least $100 for the service.
About a year ago I did a post on virtual twilight shots like this and readers argued against this kind of thing mostly because it depends too much on computer skills rather than photographic skills.
To me, this example illustrates that there are cases where virtual twilights are good enough for real estate photography. This may not always work but I think in this case it works very well. Real estate photography is a combination of computer skills and photographic skills. The particular processor that did this example is not taking new clients for the next few months because she is expanding her business. There are many other processors that do virtual twilights. Have you used any?