[wanna be a young mother]

Ok well my SIL was supposed to make the cake for Nevy's party. I had priced cakes at the bakery and they wanted $250-300 for a castle cake. I figured since she had taken cake classes and did cakes for family parties before I could ask her to do it and save some money. Well I asked her for a price and she kept saying "I'll get back to you". I understood that she was busy with work and I was fine with waiting but as the time ticked down to Nevy's party I started to get worried. So I told her before I left for vacation that I had to have a price by the time we got back. Well I get back only to find out she had already started to buy stuff!! I was mad b/c at that point I never said if I wanted her to do the cake for sure! So I was stuck with her doing it since she had already bought $70 in stuff for the cake. I again asked her for the FINAL price of the cake and she said she needed to finish buying everything before she had a price. I wasnt happy about this so I told her I didnt want to spend more that $200. She was ok with that and said not to worry. Well her party is in 4 days and she STILL hadnt called me. So I called her tonight. I told her I need a price so I can put the money away for Sat. She tells me that she has a price for me but had been putting off calling b/c she knew I would be upset. So I'm thinking ok she is gonna charge me the whole $200 but whatever I guess thats fine. NOPE!!!! She tells me I spent $150 on supplies and I am charging you $100 for my time and effort!!!!!!!!!!!!!Her time and F****** effort??? IS NOT worth no $100! Not only that we are FAMILY! Who charges family for effort??? So the total cake would cost me $250!! That is what the bakery wanted! That is what a freaking WEDDING CAKE costs! Not a childs 1st birthday cake!! Where does she get off asking me for this kind of money!!!!???? I am so hurt and in shock right now! I told her I would call her back b/c I was so mad I couldnt even speak! We CAN NOT afford that kind of money for a CAKE!! My wedding cake was cheaper than that! What do I do? My mom said to tell her to take her cake and shove it up her A**! I can order a regular sheet cake from the bakery for $90. I was thinking about just doing that until I found out she had already started to buy stuff! She KNEW I didnt want to spend more than $200! I am so lost! If I tell her I wont pay it she will start a whole family fight over it and I dont want that for my babys 1st birthday! I am just so hurt b/c she makes cakes for everyone else in the family for FREE but she is charging me for effort???? I dont mind covering the $150 in supplies but she's nuts to think I will pay $250 for this cake! I know she is mad that I am upset about her price but I cant help it!

HELP ME!! I dont want drama at my DD's party but I dont want to pay all this money for a cake! What should I do???