Relocation costs probably weren't all that steep. Whether the apartment in Sunnydale was rented or owned, flights back to the UK aren't really all that expensive, not for someone in Giles' situation. Given the house he was shown to be living in and the grounds (ASH's own house and grounds in real life) it's long been thought that Giles comes from old money. Either from his mother's side or from Edna, his grandmother. Her speech patterns suggest she went to public (private) school, which would have cost her family money, meaning they had money to be spending on a private education for her before she entered the Watcher's Academy.
If the grounds and house didn't belong to Giles, it's reasonable to assume that they belonged to the Council and that's where Giles was staying, seeing as how he was working for them full time at that point, which would then keep his living costs relatively low, since his rent would probably be subsidized.
And, as [MENTION=8860]The Thirst Evil[/MENTION] said, Giles was paid back a lot of money when he was rehired by the Council in 'Checkpoint', more than two years worth, which would have been a considerable amount of money.
As for how much Giles gave Buffy - I don't think we're talking more than a few thousand dollars, four or five at most, which would have been used to pay off the outstanding bills that were already owed and to pay for the 'full copper repipe'.
Since Buffy had to take a job at The Doublemeat Palace just a few months later to make ends meet, Giles clearly didn't give Buffy enough for her and Dawn to live on for any reasonable length of time.