At level 5, a character can afford a single scroll of Monstrous Thrall
with the XP instilled for a single HD/level creature, assuming he makes the appropriate skill checks required, for 6,325 GP (4,112 GP with Mercantile Background and haggling). Assuming a Cleric with the Domination domain and a 19 Wisdom, he only needs to make one UMD check, and can achieve at least a +13 on UMD checks (5 ranks, Skill Focus (UMD), and an item for +5 competence with the left-over cash)
Dave the Slavedriver
Human Cleric 5
Domains: Domination, X
Human: Mercantile Background
1: Skill Focus (use magic device)
2: Skill Knowledge (use magic device)
I'm sure there's a domain somewhere that nets you UMD as a class skill, that would free up a feat.
Okay, because of Skill Knowledge we get UMD as a class skill, so we plumb 8 ranks into it. UMD is Charisma-based, so for now we won't assume anything over 10 on Charisma because Cleric casting is Wisdom-based.
We need to have at least an 18 Wisdom to start, putting our 4th level bonus on top of that for thr requisite 19 for 9th level spells like Monstrous Thrall
For 2500 GP we can get a +5 competence item to UMD, and Divine Insight
nets us another +10 to our check to use it.
+26 out of 18
So obviously we can afford to cut back a bit, maybe get rid of the competence item for some new armor or a nice sauna somewhere
At most, we can cut the item and Skill Focus.
The scoll's DC to mimic the caster level is 17+1 or 18, we can easily make this, so we can assume we'll always succeed at using the scrolls.
Let's say this character, Dave, uses his scroll to permanently dominate a 1st level Expert, Jim. Dave orders Jim to work himself to the bone at his usual job, as a Fletcher, and send all the money not needed to sustain himself to Dave's bank account (Deliveries to which are sent under assumed names to assumed names, etc. to ensure no-one gets wise to the scheme).
After about 15 years, Dave has enough money to buy another scroll, and dominates another 1st level Expert, Bob. He gives the same commands to Bob as he did to Jim, and after the better part of 8 years he gets another scroll and dominates another worker, and so on and so forth.
Eventually, Dave has managed to place permanently under his control 31 workers, who toil at their jobs day and night living off just enough of their Profession income to survive. With 31 1st level workers each with a +7 to Profession (4 ranks +3 Skill Focus), he can now afford to buy a new scroll twice a year (6325 GP versus ((10+7)/2x31)x52 GP), with a little excess on the side.
This has taken course over 61 years, so we'll assume that either Dave the 1st gave the order to follow his family in successive generations, replacing the workforce as necessary, or he went Necropolitan and took the workers with him.
The end result is that with a minor legion of workers unswervingly following Dave's commands, he's on the receiving end of about 41,612 GP each year (Minus 31 multiples of living costs per annum if the workers are still living), almost 5 times what the ordinary 5th level character starts off with.
Buying up an extra pair or more of workers each year, Dave is getting richer and richer without having to worry about his workers flooding the market as with most other legion workforce ideas.