Read any book on sales or proposal writing. Go on a sales training course. One of the first topics that will be covered is features, advantages and benefits. Ultimately, any proposal whether verbal or written down has to persuade someone to take an action and this is done by demonstrating the benefits of taking that course of action.
In a smaller sale, especially with products it is often easy to list benefits, but with larger proposals you will have to search harder to understand the clients business problem and explain the benefits of your solution that is, how effectively and efficiently your solution solves their business problem.
Your solution should demonstrate not only value in price, but in the services that you offer. Value is the balance of cost versus benefit, therefore you need to be explicit in highlighting the benefits to the client.
Often when a contract has been lost, the client will tell you that you were too expensive but no-one ever gets told that they won because they were the cheapest. It is the best solution that usually wins, the one that offers the best benefits. This may be the most expensive solution, but it still may offer the...