I've made a web page with table of levels' exp/gold costs sorted by exp ascending, taking into account that avatar levels can be made only after 4x40.
So if you're unsure what to level next and interested in maximum sponge efficiency, that page could be helpful.
Link: http://rds1983.github.io/SlothMUD-LevelInfoParser/
Btw, I've got values by parsing this page http://www.slothmud.org/wp/welcome-to-slothmud/the-library/expgold-tables. I know there are some errors, but they arent important.