Harold Ancell was good enough to share his own research into how well the Dursleys did financially by Harry's enforced residence with them. Here are some of hisremarks, including a spreadsheet of the child benefits:
Back in 2006 I had the same thought about how the Dursleys would be pocketing Harry's government benefits and I researched what those would be... I did work up in the attached spreadsheet what the U.K. Child Benefit would be, figured in various ways (value at the time, adjusted to the April 1998 price level, cumulative, etc.).
Bottom lines are: by the time your story starts, he's receiving around £9/week, £480/year in 1998 pounds. Up to that point they stole around £6,000 in 1998 pounds (!). And that's just the Child Benefit....
If you find this of interest or use, I'll see about the guardian's benefit; if you can't read this spreadsheet I can convert it into a form that you can read.
You're entirely welcome to spread the spreadsheet far and wide; I originally published it on the fanficauthors.net forum, but that was taken down due to inactively :-(. I found the drafts of my original postings and I'll send them to you after editing them, they include details on the Guardian's Allowance.
Here's a nicely formatted pure ASCII text version of the spreadsheet that I made for the forum posting. It rounds the display of the weeks each benefit level was at but is otherwise the same:
UK Child Benefit, for second or later child
actual April 98 prices wks pr wk pr wk Apr-79 £4.00 £11.98 actual Apr 98 pr actual April 98 prices Nov-80 £4.75 £11.12 total in period cumulative, per year Nov-81 52 £5.25 £10.97 £273.75 £572.01 £273.75 £572.01 at Dursleys
*For ages 16-18 there is no benefit is the child is "receiving advanced education or training under a relevant training programme" (http://www.hmrc.gov.uk/manuals/cbtmanual/CBTM08010.htm); it is not clear is NEWT level Hogwarts study would count.
Certainly; the below is from my first posting to the previously mentioned forum, edited appropriately:
Raising Harry, a profitable enterprise?
When doing some research, I realized that the Dursleys probably turned a profit in raising Harry.
During Harry's lifetime the Benefit changed so that the amount for each additional child is lower than that for the "first" child. A House Of Commons Library research paper has at the end a chart of the child benefits, actual and adjusted to April 1998 prices, see www.parliament.uk/Commons/lib/research/rp98/rp98-079.pdf or http://tinyurl.com/lf8zsx (and I used that for the spreadsheet, April 1998 of course being particularly convenient; at the time I assumed it would be around the end of _Harry Potter and the Final Book_ and as it turns out it's *just* before the final battle on May 2nd).
Note that there have been tax credits of varying sorts, but they seem to be means tested (and currently do not depend on the number of children), so without knowing Vernon's income it's not clear what we can do with that.
There is also a Guardian's Allowance that the Durselys would be able to claim for Harry....
However, I couldn't find any historical data....
Ah ha! I just found a spreadsheet at the U.K. Institute for Fiscal Studies at http://www.ifs.org.uk/ff/ga_csa.xls with the historical rates for the Guardian's Allowance (going back to 1948!), and it checks with the current Technical Manual's 2004-10 rates. And they are as I guessed reasonably generous, but the year by year relationship to the Child Benefit isn't one to one. Usually at least one pound a week more, but not quite that for a period of a few years before your story. Generally a couple of pounds more, and at the time of your story it's a handsome £10.85 per week on top of the £7.25 (soon to be £7.50 in October) second child's benefit. Going forward for the period in question, each April it increases by 0.1 or 0.05 pounds.
This is not surprising, seeing as how there are very few orphans compared to the Child Benefit going to the mother of every single non-alien child in the U.K., which of course means JKR knew about as she was writing the first book as an unemployed and unhappy about her benefits single mother.
Some more research today finally resolved how the Guardian's Allowance works WRT everything else (see http://www.neath-porttalbot.gov.uk/default.aspx?page=435 or http://tinyurl.com/n3lnyv for what resolved all this): it's non-taxable and as far as I can tell it is means tested only if someone is on Income Support (general welfare, which the Durselys most certainly would not be on, see http://en.wikipedia.org/wiki/Income_Support or http://tinyurl.com/l4tqou ), and only then is it deducted from that particular benefit. Otherwise I don't believe there is any means testing or adjusting, so at the time of the story the Dursleys are stealing £18.1 pounds a week (!!!) from Harry or £940 a year. That's no small amount.