The most cost effective way whilst still acheiving a fair amount of XP per hour is Serums 207. They need level 15 herblore to make and give 50xp per, they also make twice as fast as a normal potion. To make them you must have started the shades of mor'ton quest. I calc'd this as an option for me from 92-99 and it would have cost 55m. So you are looking at around 110m for 99. This xp was to slow for me so im doing a more expensive option, but this is deffinately the best option if on a serious budget. If you have 50m to spend only, you could get up to extremes doing serums. From 91, always try and do effigys for people aswell as that 30kxp a day does save a fair bit of money. all though you do have to fight to get the assist.