First thing to do is figure out exactly how many servings they are ordering, so you make the right sized cakes. There's too much difference between 50 servings and 70.
If you make a cake big enough to feed 70, than you should get paid for every serving. Like wise if your cake is small why should the client pay for 20 more servings than you gave them?
Second thing you must look at is, what is a reasonable price per serving? Go to a restaurant and order a piece of cake........see what they charge you. Go to your local bakery and find out what they charge per serving. If you charge them $120. for 70 servings that's $1.71 per serving. I can't think of any place in the world that would charge that little. Even a cupcake with-out any decoration on it costs more.
So start doing the math yourself.
How long did it take to make that cake? (I'd guess 8 hours for a beginner, going to the store to get the ingredients, baking and decorating.)
How much did it cost you to buy the ingredients? (I'd guess it cost you around $50.00 to make that cake)
Where does the math take you:
$120.00 minus $50.00 in ingredient costs gives you $70.00 of profit.
If you spent 8 hours making that cake, $70. divided by 8 hours, equals $8.75 per hour of payment.
You also have over head., since you can't post this question unless you are a legal business.
My guess is your overhead is $2.00 per hour.
Than ask yourself if your work was worth $6.75 per hour? Even a 16 year old gets more pay per hour than that at a minimum wage job in the US. So regardless of your experience decorating cakes, is your time really worth that little pay?