The FHA Construction Loan program allows for the trade-in of a manufactured home in lieu of a cash down payment. You may even be able to roll in your closing costs.
Your home must either be debt free or have significant equity. If you have a loan on your home please find out the balance and then check the following site to find it's value;
You will need to know the manufacturer of the home, the make/model, the year it was built, dimensions, options and the general condition of the home. The value you get is not necessarily the price your builder can pay you for the home. Other factors may contribute to it's final value.