I made this cake earlier in the year.
It's a 9x13x4 WASC with vanilla bean butter cream and filled with raspberry puree. I did the letters in fondant. The car is made of rice cereal treats and covered with fondant. The figure was made of gumpaste. It probably took me a couple of hours to make them, as I wasn't sure how I was going to make it work. And the cake probably took 2-2.5 hours total.
I charged $150 at the time for the cake. I've now been asked to make the same cake for a birthday minus the figures. I was thinking all butter cream, no fondant. So I would pipe any writing, and probably flowers. Would $100 be too low do you think? I'm not sure what the car and figure would have been valued at. I'm sure I probably undercharged at the time. I had made another identical cake at the same time for someone else that was all buttercream, writing and scrolls, and charged $100 and was told that I had given them a really good price.
A 13x9x4 is 50 1x2x4 servings. For me, a butter cream cake is $3 a serving. So the cake would be $150 for me with basic decorations.
To me, when it comes to fondant decorations, if when you leave them off the cake is very plain I don't really feel there is an up charge. Most of the time you would have spent decorating the cake (which is factored into your costs) is being used when making the figures. I may tack on a little extra for the fondant i used, but in this case it wouldn't be enough to price them separately for a large amount.