Have you already got a pan 9 x13 or close (will make about 60 servings). If you make a separate cake that size in addition to your 8-10-12-14 stack, then you get 300 servings without the grooms cake. This cake would be the same cake and icing but with only a simple shell border, that stays in the kitchen but gets cut up and served out with the rest.
Please do NOT get into the mindset of "not all people will have cake" --as a cake maker you WANT to sell cake. If there is a grooms cake, some people will ask for both. It's not a good idea to count the grooms cake into the "wedding cake" total. You can't just say to people "take one or the other". The grooms cake is frequently chocolate and lots of people will go for that if they have a choice.
So talk all this stuff over with the bride. She is either a close friend or family member--easy to talk to. Get her to find you a picture of what she wants for decorations if possible.
As to price, try to estimate how much time it will take you to do the work. Use the baking times in the Wilton cake charts and remember you will have to use each pan twice. Plus mixing icing and icing all those large tiers. You count your time at $10.00 per hour, plus the cost of cake and icing, plus something for having to buy some pans. You will see how that gets you close to $1000 even without any fancy flowers or such.
Here is the table with batter and icing amounts.
And make sure you buy the large package of Magic cake strips and use them. They are the right size for these pans.
Final piece of homework, read ALL the links from here
I was 19 when I made my first wedding cake. You will do OK.