You don't have to buy the cake tins. Almost every town has at least one 'sugarcraft' shop and most of these offer a hire service. Not only will you save money, you'll also be able to get plenty of free advice to help you make the perfect wedding cake.
The only problem with tracking down sugarcraft shops is that they don't have their own classification in Yellow Pages. There are a couple listed in my local copy, however, (both offering tin hire) under 'Cake Makers & Decorations'.
Hoping this helps,
Chris