So it used to be that maxing TH8 completely (defense, walls, BK 10) was highly recommended before upgrading to TH9, however, ever since TH 10 came out along with new troops for TH 9 - such as Healer lv 4, Balloons lv 6, it is no longer recommended to max TH8 completely before upgrading to TH9.What you need to do is to max all the defenses, get BK to lv 9 at least, and have at least all lv 7 walls (purple). Because with pink walls you would be attacked quite often as it is perceived to be an obvious mark of "premie". Purple walls is the perceived boundary between "premie" and "well developed" bases at the time of me writing this guide. Also because you want to max your 2 heroes ASAP, so sitting in TH 8 waiting to max walls with sleeping heroes will slow down your progression.You should go into TH 9 will full storage of DE and elixir - DE for hero upgrades, and elixir for offensive upgrades as well as the 2nd drill.
According to Wikia, the total cost of elixir upgrades for TH 9 is about 85 million (84,948,550):