The £50 per annum after Mr Bennet's death would be interest (at 5%) on the £1000 that Lydia will get out of the "her equal share of £5000 on the death of both her parents." There is £5000 available to the Bennet daughters -- £4000 from Mrs Bennet's fortune and another £1000 settled on her (out of Mr Bennet's assets, I believe) when the Bennets married. The amount that goes to each daughter was not previously determined, but now Lydia's share will be specified as £1000.

The £10,000 is Mr Bennet's speculation on what amount of money it would take to bribe Wickham to marry Lydia. Mr Bennet does not believe that Wickham had anything like that much money, especiall since Wickham has left debts in both Meryton and Brighton. So Mr Bennet assumes, at this point, that Mr Gardiner has somehow or other managed to find £10,000 to settle on Lydia (for any children in the future) -- the income on that £10,000 would provide £500 per annum if invested at 5%.