Well my plans when I go for 99 herb will cost ALOT more than 40 - 50m, but I'll share it with you as you might like the idea and save up to do it this way... Last time I added it up, It cost 70m from 90 - 94. Make an equal amount of all the extreme potions(attack, strength, defence, magic, ranged), the formula, incase your not good at math is- Add together all the xp from the extremes, this is 1200 xp per extreme set. Now take the amount of xp until level 94 and divide it by 1200. This will give you the amount of each extreme potion to make. This will get you as far as 94. Now if you started doing this method at level 88, you would have enough supplies to get to 99 with overloads. However, from 90 - 94 only gives enough extremes to get 97 herblore with overloads I believe. And 94 - 99 will cost you over 100m, however you will have nearly a lifetime supply of overloads.