ok work out how much money you think the business will make each year, then work out how much mone they are going to have to invest at the beginning to set the business up. divide the money you think it will make by the money they need to invest.
Eg they will make $40K sales in the first year, there will be $35k expenses in the first year so te business will make $5K
to set up the busiiness they need to buy xyz so will put in $100K (this is called equity)
5/100 = 5% return on investment for the first year.
hope that helps