Wow, that's a lot of cake! Here are my thoughts for whatever they're worth!
First, if you've never worked with fondant, this wouldn't be the best cake to start off with. Then again, you have some time to practice before August rolls around. Does she need this much cake, or is she just in love with the design? If it's more the design, I'd make the the upper tiers from cake dummies to cut down on some of your work.
The pricing question is difficult to answer. Most charge by the slice for this type of cake. I would say that you might want to charge a modest amount for this cake since you're not real experienced using fondant, but if you could somehow talk her out of the fondant and just go with buttercream I'd say no less than $1.50 per slice (for buttercream) for a regular customer, I'd probably go as low as $1 a slice for my best friend. I know I'll probably stir things up a bit by saying that! If she insists on fondant then you're prices should be more. Fondant takes a lot more time than buttercream and depending on what kind of fondant you use it can be very expensive. So, maybe factor up your costs and see where you're at.
This cake says it serves 332 so if it were me, and I was doing the fondant thing, I'd probably charge a customer $830 for this cake ($2.50 per slice), a best friend I'd charge $500. I'd charge another $100 just for the gumpaste flowers, which I know is probably not enough. If this were all buttercream I'd charge about $581 to a customer and maybe $332 to my best friend, but still $100 for the gumpaste flowers. Also, if you're going to have to buy pans or other supplies you need to consider that too.
I don't know how much help this was, but I'd definitely sit down and discuss the price of this cake. She may not realize how much she's asking you to do. Maybe if you price it all out with the supplies and give her an estimate of the amount of time you'd be putting into this cake she'd be cool with paying you a reasonable amount. If not, have her chose something considerably less complicated.