Just having a look now and there appears to be a few versions of the AD480 (free, basic - £2.99, pro - £10.99). The basic looks like it has a few more features than the free, so possibly worth the £2.99. I assume the free one is designed to demonstrate that it sounds good and entice you into spending a bit more?
I also found VocaLive, which looks a bit gimmicky, but is free for the basic functionality and then you can make in-app purchases. I'll have a go with this and see what its like.