If you do know exactly what you want, do not hesitate in finding the cake maker who can do this for you. Generally, they require at least 6 months notice, and some months are typically busier than others so will call for more notice to be given. Try to give your cake maker as much information as possible about your ideas so they can give you an accurate quote. If you have expensive taste and love the idea of cascading sugar flowers, intricate piping details or tiers that reach the ceiling, be prepared to spend. The more time consuming and detailed the design is, the more it will cost. Don't forget to ask about delivery charges and equipment hire as you will need to factor these in also.
All in all, make sure that you have the cake that YOU want, but also that you've set an affordable budget. If you don't mind compromising on specific details to reduce the cost, that's fine but make sure it's a cake you're going to love!