Putting your parcel together is a lot of fun, but remember that it won't be going anywhere without a nice big stamp. Here are a few UK prices to help you get a rough idea of postage costs.
A small parcel (45cm x 35cm x 16cm), weighing up to 2kg, costs £2.80 to post second class.
A small parcel of the same dimensions but weighing 1kg costs roughly £8 to send to Europe and £13 to post elsewhere.
A small parcel of the same dimensions and weight costs roughly £14 to send to Europe and £20 to post to the rest of the world.
The more you put in your parcel, the heavier it'll be and the more you'll have to pay in postage. Small, light objects such as tea, socks or cards are ideal for keeping your package nice and light. If you decide to track your parcel or opt for it to be signed for on delivery, costs will be higher.
Please think carefully before opting for an international swap. Prices for posting are considerably higher, and there have been a few cases in the past where people have had to pay a customs charge to receive their package from abroad. If in doubt, just sign up to swap within your own country - that way, there should be no nasty surprises at the post office.
There's lots more information on the Royal Mail website. You can even enter your parcel through their online price finder to see how much it'll cost.