Halloween can be expensive, but that is only if you let it! As it becomes more and more commercialised, so do the Halloween fancy dress costumes. The good news is that with all this competition around, there is plenty of competition and so if you are prepared to shop around you can find many good deals and cheap Halloween costumes. This is all assuming that you are looking to buy and don't want to make your own Halloween costumes.
Even more good news for you is that I have done a little of that research for you and so here are my top Halloween costumes for under $20 or £10 (I also get quite a few people from the UK reading the fancy dress ideas blog)
Child Little Vampire Boy
This vampire costume costs around $15 - $20 (£7 - £10) and as you can see makes a perfect Halloween Costume. There are plenty of Child Vampire costumes at this price range.
For the Girls, I found this Child Witch Costume which should only cost you around $10 (£5) ans so I think is a real bargain.
Many thanks to Jokers' Masquerade Halloween Costumes for the cheap fancy dress ideas from the UK.
Your other alternative would be to make your own Halloween costumes, but be careful as I have found, sometimes buying the materials can cost more than if you were just to buy the Halloween costume in the first place!
So there you go, unless you are looking for deluxe Halloween costumes, there is no need to spend a fortune at Halloween.... save it for Christmas!