They very rarely start with a big up front fee. Often they start very small.
If you take the bait and agree to host their money they often ask for a small fee (say �50) to pay for "admin" or something like that.
If you are stupid enough to pay that then they come back and ask for a bit more (say �100 or �200) for some other mythical reason (local tax, export fee or whatever).
You have paid the first �50 so agree to pay the next figure.
Then of course they come back for another amount (�500 or �1000) maybe to bribe a local official or whatever.
Your are already out of pocket by �150, so you think what the hell, you pay the next amount.
Then of course they come back with another amount (�2000 or �5000 or whatever).
By then hopefuly your alarm bells are ringing and you cut your losses.
But some people are greedy and they keep paying out, and some people have last THOUSANDS to these ba5tards.
It is a bit like catching a fish, you get the hook in their mouth then you reel them in slowly.