The "Cost to Exchange" is the bottom line. It's how much money you actually have to find to complete your move. For example, if your current property is valued at £120,000 and you want to upgrade to a house that costs £175,000, then your "Cost to Exchange" is £55,000 (£175k minus £120k). That's assuming you get the full value for your current home and that the seller accepts your offer, of course.
If you get less for your current property, for example by selling it to Delma Homes Ltd. at £95,000, and you put in an offer at £150,000 on your upgrade home, your "Cost to Exchange" is still £55,000.
These figures are, of course, just for illustration. They do not necessarily represent what you may or may not be offered for your current property.