When I was a bride on a limited budget, the first quotes I got for wedding cake about gave me a heart attack too, so I know where you're coming from. However, since then I've made several wedding cakes professionally, and I understand a lot better where those figures are coming from. Wedding cakes really are a lot of work and can be stressful enough even when it's not your own wedding.
Here's some points to help you discern whether it is worth it to you to make your own:
1. How experienced a baker are you?
2. How accomplished a decorator?
3. Are you willing to have a small, simple cake or do you really have your heart set on a sugar art showpiece?
4. How well do you handle stress?
If you are already and accomplished baker and decorator, a wedding cake, even a fancy one, is certainly doable, but no matter how skilled you already are, it will take up a great deal of your time before the wedding and definitely add to your stress levels.
Whether the cake would be something you can look back on fondly as putting your special touch into your special day or something you hate to look at because it only reminds you of how much stress and misery it added depends a lot on your personality and on how much you care.
Now, I'm not normally a happy-go-lucky, roll with the punches person, but when my friend who had promised to arrange my wedding flowers for me cancelled three days before the wedding, I was just fine. My mother nearly died of stress as we sat in the reception hall the night before the wedding with buckets of roses, a leaf strewn floor, and spirals of ribbon all around, but I could laugh about making my own wedding bouquet at midnight before the wedding. Why? Because I didn't give a darn what the blasted thing looked like. If you have the same attitude towards your cake, you'll be fine. But if a beautiful, professional looking cake is very important to you, then I'd be really hesitant to do it yourself.
If you do decide to make your own, make sure you learn about how to securely support tiers so your cake won't fall apart under its own weight, and remember that "rustic" iced cakes are very trendy and "in" right now (totally plain, smooth sides are a heck of a lot harder than they look!).
I hope that helps you. Good luck