I love round cakes, personally. I have never cared for sheet cakes so I don't do them...
I guess the standard response is 'Call your local bakery to get an idea of what the going rate is'.....
And don't undercut the bakery... you are offering a more customized service then they are ( I swear, I am typing this word for word from the responses I got when I asked about price - I am not very original! )
I will tell you that I was real worried about sounding "too" expensive in the beginning and that all went away the first time I did a cake for peanuts... 6 hours, it took. So I figure I made about $2 per hour, after expenses. Not a good feeling 
I have now decided I would rather people order from Costco then bust my tail for nothing!
I charge $30 for a 12x18 sheetcake (feeds 36+). and $25 for an 11x15 (feeds 28+). People on cake central think I'm low, but my customers appreciate it.
This may seem pricey but I feel like my time and extra effort I put into making the cakes is worth it.. I've been doing a ton of CCC and I've been getting 40.00 for a 24 Ct. decorated cake..I've not gotten one person that has complained about the price.. The least I've charged is 35.00 and that has been for my co-workers 
We do so much for each other I feel that it just comes back around.. Plus you can't get most of what is done on CC at Walmart.. So I look at the work of everyone here as a Custom Service.... That's just my two cents on it..
Cassandra
Thank you everyone for your answers. I decided to charge the girl $30 because I am still somewhat new and trying to get my name out there and I appreciate her wanting to use me for her cake. Later on when I feel that I can up my price I definitely will. I did however talk to a girl I work with who said that she pays $50-$60 a cake from her cake lady so maybe one of these days I can ask the same for mine! I appreciate all your help! Have a great day! 
I've been doing a ton of CCC and I've been getting 40.00 for a 24 Ct. decorated cake
Cassandra, I too charge $40 for a cupcake cake whereas I charge $25 for the same servings of a sheetcake. Although a CCC makes things easy for the customer, I fell it makes things HARDER for me. So, I prefer they choose a sheetcake over a CCC.
Tugboat: I think you're charging a fair price.
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