My 3D carved cakes are priced between 4 and 5 dollars perserving, depending on design. So for 50 servings, it would be a minimum of $200.
Working inside a hole like that is a PAIN!!! At least $200
You have to consider the time spent on that project. To get all that detail and everything will take you hours. If you charge $200.00 for that cake and lets say on the low side your supplies run you $50.00, that's $150.00 left over for your time spent baking, decorating, delivering, setting up, and driving back to your shop. All told that could come out to 10 hours minimum!
Are you okay with working 10 hours and getting paid $15.00 an hour? If so, then $200 for that cake is a good deal for both you and your client.
For a 3D cake that takes 10 hours from start to finish, I'd charge $350.00 minimum. If the market in your area will bear that out, I say you should charge that. If you're uncertain, split the difference and go $275.00. If you're not sure what your market area will support, call some of the local bakeries and ask what they'd charge.
Remember, our time is just as valuable as any other professional out there. The reason why they come to us is because 1.) they can't buy it off a rack, and 2.) they can't do it themselves.
10 hours??? Seems like a lot...