Maybe demonstrating value is hard - there's lots of proposals out there that never demonstrate good value.
What is value? Value is showing the customer that what you are selling is worth more to them than what it costs to buy it.
So why do so many proposals stop at just showing the client what it costs to buy it? Go that one step further and show them how much money you can save them or how much time you can save them. Put a value on it (get the information from the client, if you know them well enough) and work out the return on investment.
Showing a client how they can make or save money could be the key to keeping you in business. Use it.