On different phases of the game try to watch AI cash & funding from turn to turn (use the thiefs guild). You will see that often AI receives plus 10000 gold to its total income !!!
Other day, Fabrice said that
The way I see quite good is for example H3 AI. If you would give it at least 5000 gp a day- that would make map nearly imposible to win in most cases.The AI has been done and tuned with the help of Steve Fawkner, and the result is quite good
While H5 AI even with 10k - sucks.