Personally, I think that these items aren't worth the prices usually asked, but Play have a couple of good deals - very useful if you son has asked for them for Christmas, like mine has!
The deals are as follows:
1. the battle pack is reduced from £19.99 to £9.99
2. when you buy the battle pack, you get the starter pack for £4.99 (reduced from £12.99)
This actually seems like a good price for what they are ;)
You could combine this with the arena for £19.99 at Argos if you want to save a few more £££