I agree with NyaChan about the meat sauce. Maybe, budget allowing, grill up some Italian sausages and slice (or crumble) on the side if people want to add it to their marinara. I understand that the meat takes on flavor when cooked in a sauce, but we're looking at a tight budget.
Or sautee some cubed chicken breast with a basil pesto (or spiced as you like) instead of the sausages. Chicken will work well with both the alfredo and marinara.
Mushrooms are a good alternative to meat and cheaper, too. Maybe do a side dish of mushrooms (sauteed in a savory broth) instead of the above suggested sausage/chicken.
What are you planning for the sauteed veggies?*
Pasta? I think I would go with a spaghetti and a penne or rigatoni for variety. The pasta is the least expensive, so go with plenty of this.
Maybe add a basil pesto to your table (small jar about $5-6, served in a bowl with a small spoon) This would work well for a 3rd sauce. You won't need much because of your other sauces.
Appetizers? Depends on your schedule. You already mentioned bread and olive oil. What about serving that for the appetizer? Maybe slice a few tomatoes splashed with balsamic . . . what isn't eaten during the appetizer phase can always be brought to the dinner table.
Good luck! I really think you can do this on a $100 budget.
*You can take this for what it's worth (my own personal preference
) Please stay away from green bell peppers! In my opinion, they are overpowering and the other veggies soak up the flavor of the green bells and that's all I can taste. But that's just me.
eta: I just googled "How much pasta?" and it seems like the general consensus is 1 lb. per 4 people if the pasta is the main course.