Interesting thing to think about - I did a search on the cost of asylum seekers and found this quote:
"And, according to a recent Home Office study, migrants - including asylum seekers and refugees - are far from being a burden on UK tax payers. In 1999-2000, migrants in the UK made a net contribution of approximately £2.5 billion, worth a penny off income tax."
(from Wigan Council - Asylum Seekers The Facts - a nice little FAQ with some interesting info in there)
Hm, maybe it's worth ignoring the tabloids on this one when we start getting worried about costs...