really depends on economy of your server but i think i have developed a way to maximize your profits on plat although it may take some time:
only get:
1. sigils
2. backpack space (only as much as you need)
3. bank space (only as much as you need)
personally i have spent most on bank slots as they give more space
backpack only has to carry the stuff you need with you at all times (idols/pots/miscellaneous/etc)
keep the rest of the plat and sell alt books for 4k (some people might think thats a rip and rage at you, but its actually an ok price)
id say that the average price of alt and rebs would be 3k and 6k but you can raise the alts by 1k for maximum profits because the difference isnt that big
dont raise the rebs by that much tho because its much more noticeable when you raise the rebs from 6k to 8k
keep the money you earn from selling these and either just use it for your own purposes or purchase super elix from other players for 3.5k - 4k
thats what i do and it works just swell

sometimes the really stingy players might rage at you but youre not doing anything wrong by "overpricing" anything
if they dont wanna buy they dont have to