There are a few answers already, but as the question was bumped, I'll share a bit of my experience with "sleeper" chars. First, most of my suggestions only make sense for a character with a decent magic level, that is the character was offline training for a few months or used a few durable training weapons.
Regarding profit, head over to a great table in this answer: How can I make money with low levels?. TL;DR Blood Crabs, Banuta Apes, Pirates, Cults. Another option is to farm 5000 minotaurs and wait for double loot event. For instance, you can clear full spawn of Darashia piramid (~100 minos) and x-logout at a safe place at the bottom. If done daily, it takes about 2 months of 2 minutes runs to complete, but the char is not training at that time...
Regarding full-waste solo exping I've tried many places, and I've found the Edron Old Fortress -2 as the best combination of safety and experience. Using SDs with Great Energy Beam and Fire Bombs as on the screenshot (safety and easy beams) + Exp prey on Hero and Damage prey on Squire with 70 lvl MS I had raw exp rate of 350k/h and a grand total 1.7kk/h (double exp event) while loosing ~250k gp/h. I see no reason why this strategy wouldn't work for level 50.
Regarding spawns previously mentioned, like Ancient Scarabs and Water Elementals, the exp rate was comparable or worse, but the risk of death raised like 10 times.
There is also a nice FACC place for level 28+ mage with magic level... 6: Memory of a Carnisylvan yielding 200k/h raw exp . Summon 2 Demon Skeletons, which are immune to earth damage and heal them from time to time. Burn the ground with runes and run (in circles) away from your summons which are followed by Carnisylvans burning till death. (With a 60 lvl mage I got raw 250+ k/h with an alternative strategy involving own fire power).
Source: own experience and own contributions to the linked articles :)