It's a policy based on false numbers, no matter how well presented.
Both the Financial Times and Guardian are pointing this out.
FT
He also claimed that the changes his party was proposing would save UK taxpayers £234bn over the lifetime of the migrants.
The figure appears to come from a February report produced by rightwing think-tank the Centre for Policy Studies, which subsequently said the fiscal data contained within this report was the “subject of dispute”, meaning that the overall cost estimates should no longer be used.