The cost is the same either way, if you put stamps on yourself you need to calcuate the amount by weighing the parcel and checking it's size as postal rates vary according to the size and thickness of your parcel. If you do want to do it yourself you can get the prices via the royal mail website. Do beware however if you don't put enough postage on, the person recieving the parcel will have to pay the extra, as well as an extra £1 handling fee, which isn't fair on the person you're swapping with.
You may find it easier to use your local post office until you get a feel for it (then you can opt to print paid postage labels on line by setting up an account if you wish, which is easier than buying stamps and making up the right amount).
And, if you've not yet read the advice in the Info for new users, 2nd class is what most of us use, as long as we send promptly after agreeing the swap.