I have someone who wants me to make a tennis racket cake for them. It would be actual size of a real racket, cake made from scratch, homemade buttercream and marshmallow fondant. I don't know how much to charge as I usually don't sell my cakes. I know this is going to be kind of involved, so I definitely want to charge for my time. Here are my costs: (I use mostly organic ingredients)
Buttercream: $8
Fondant: $5
Cake & Filling: $15
Board: $3
--------------->Estimated total for ingredients $31
I was thinking of charging $100 for the cake, what do you think?
I think that seems really reasonable and I don't see why they'd have a problem paying that for a custom cake WITH fondant
That is no where near reasonable, it's ridiculous. Ridiculousy low. 
So, ingredients cost= $31.
Your time then, for all this work, attention to detail, baking, making fondant, decorating a board (oh more hard costs there), is worth what, $69?
Right, so do some more research before you spout off a price to someone, and you end up back here complaining that cake-ing is too much work for the amount of money you're being paid. It's a lot of work, but a lot of us get paid quite well for it. We never allowed ourselves to be taken advantage of, because we did our research before beginning to sell. This is a good place to get a feel for pricing. It's very sad to see people practically giving away cake for free and then wondering if this whole business is worth it or not. 
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