I assume you're talking birthday type cakes and not wedding cakes?
The question is difficult because there are so many factors. The first question should be what is the avg number of servings in our (non-wedding?) cakes? Because most of us price per serving. The avg cost of the cost isnt' good information if the size of the cake is unknown.
BC or fondant? Does the price include gumpaste work or not? What about RI tiaras 'n such? Layered or single sheets? Torted? Filled?
$50 for a cake that serves 100? Great deal!
$50 for a cake that serves 10? Wow, expensive!
It might be better to know avg price per serving and avg servings per cake sold. A flat avg price .... just too many variables.
As part of educating the client, though, an idea would be to reply to their "$50 budget" with something like, "Oh, you're looking for a smaller, simple cake, then. No problem! What I can do for $50 is a 2-layer round 8" that serves 20 with plain BC and a few BC roses."