I think we can vaguely peg his Marvel earnings to what he paid for the house. It was marketed for £2.75m in Feb 2012 and he bought it for I think a little less than that. That is a fairly hefty mortgage for a single jobbing actor - assuming a £2m mortgage you are talking about £12,000 a month.
The job is not exactly secure, and the money may be a lot in one year and not much for a couple of years thereafter, if you're doing theatre work and what have you - so it's not very easy to service a £12k a month debt. I therefore assumed that he had already banked enough for the deposit and knew from his Marvel contract that he would be able to pay off the rest once he had done all the Marvel films, or at least get it down to a feasible level (say £500k, which sadly in London is the mortgage on a 2 bedroom flat). So his net Marvel earnings after everything else - tax, agents' fees, expenses - would have to be at least £3m. Say £2.25m for the deposit and paying off a big chunk of the equity, plus £750k to live on over 7 or 8 years (2010-17).
I reckon as a self employed person that you take home about 50% of what you earn when you take into account tax and deductible expenses. Maybe a bit less than 50% as you earn more - or maybe a bit more if the income is smoothed through tax efficiency. But £6m overall sounds about right to me. Maybe £750k for Thor as he was an unknown, then £1.-1.5m for Avengers and each of Thor 2 and 3.