for this you will need high enough mining to smelt eternium, fel iron, felsteel.
i dont know about other servers but on mine i can get 2 stacks or eternium ore for 70g and the 3 stacks of fel iron ore for less than 10g a stack. im saying take ore because it is usually cheaper but if thats not the case go with bars
what i do is take the 20 eternium bars and 30 fel iron bars and smelt them into felsteel. even if i buy the mats i make a 100g proffit. sell them in the 10 bar form because believe it or not people will buy the 10 stack for 200 over a single bar for 21.
this is also great if you need felsteel because if you can wait a week of you making felsteel and underprice the competition you should have 1000g in 5 days while the felsteel market has depreciated vastly from 21g-14 . but the problem with this is you do this one week than skip a week or two because the eternium economy will zip up in price which will make it nearly impossible to make a proffit off of felsteel.
i have ruined my felsteel economy on my server 4 times so far and each of those weeks i make 1000g so good luck!!
(my bigest accomplishment was taking felsteel from 23g per bar to 8g)