There are three ways to set your prices for this sort of business: (1) Charge a flat fee for your services; (2) offer a cost-plus contract in which the client agrees to pay all agreed-upon expenses plus a specified percentage; or (3) provide an all-inclusive contract in which the client sees only the bottom line. Whichever way you choose to establish your rates, it should be based on an estimation of how much time you will need to devote to the job. If an event is going to require 40 hours of work by you, and you determine that you need to receive $25 per hour to pay the costs of running your business and generating a profit, then your contract should either (in this example) consist of a flat fee of $1,000, a cost-plus figure that produces about $1,000 for you, or an all-inclusive amount that is marked up by $1,000.
In order side, you also have to make certain that your contract is properly drawn. If the client makes a change to the requirements those results in a higher cost, the contract should be written in a way that allows you to increase your charge accordingly. Similarly, you should seek written contracts or bids from your suppliers with full details about their responsibilities and products. Don’t go out of your way to lose money, but you might want to take on a job or two on a break-even basis or at a minimal profit when you’re getting started to help you build a reputation and some references.