]How did you decide to increase Bingley's wealth by saying that half his fortune would pay off Mr. Hurst's debts of 80,000?

Actually, I didn't increase it. According to P&P (Chapter 4), Bingley's inheritance was about 100,000 pounds, and his sisters "had a fortune of twenty thousand pounds." (It's not clear if they each have 20k, or have 10k apiece. I interpreted it as the latter). I set Mr. Hurst's fortune at 30K, which, combined with the interest of Louisa's 10K, gives him an income of two thousand a year. So, his 30 + Louisa's 10 + 50 (half of Bingley's estate) = 90K, which would be enough to bail him out of his 80K debt but leave a principal of 10K to live on. Invested in the 5 percents, that's an annual income of 500 pounds – which would hardly support him and Louisa in the lifestyle to which they've become accustomed.