Most builders nowadays will leave it in a reasonable condition so you are unlucky.
Well, digging is free. Just start at one side and do a bit at a time. Take out the builders rubbish as you go. Some will need removing but the smaller stuff can often be used as hardcore for paths etc. Patience is a virtue in gardening and is necessary if you can't throw money at it.

But you could get a couple of quotes for having it levelled - it might not be as bad as you think.