Call of Cthulhu plays in the real world, so you can just use any online resource about historic salary levels, which will also give you more detail by country and other criteria impacting wages than the space in the rulebook could.
For example, here are salaries from the 1920s in the US. Based on this
- Unskilled labor per hour cost about 20 to 70 cent per hour in the US, depending on city and state
- Common laborers made about $47 per week
- Accountants cost $15-30 for junior, $25-50 for senior accountants per day of services
- Senior white-collar labor like an office manager made about $400 per month
- Airplane pilots made about $550 per month
- A dentist made about $7,000 a year, or $580 per month
- A college professor made $2000-3000 for an assistant, $3000-6000 for an associate and $4500-9000 for a full professor, per year
So, you can expect a surcharge when hiring people only for the short term vs employing them longer term.
Rules Mechanics of the "Spending Level"
If you are looking for the rules impact of Credit Rating on available funds, look at the table II: Cash and Assets on page 47 of the Keeper's Rulebook. It specifies how much cash a character has on hand, based on their credit rating, and how much assets they own. Cash (p. 46) is what can be directly wired to them or is at hand, within a day or so. Assets include things such as a house or a car, and would take a longer time to turn into money, by selling them. Spending Level is just an arbitrary amount below which no record keeping of spending is required, if that amount is spent on occasion within a day. The keeper may demand liquidating assets if a player tries to exploit this.
The spending level exists purely to ease the flow of the game, no one wants to track every penny.
And the following passage, which also clarifies that Spending Level is meant to be per day:
In theory a player could spend an amount just below their spending level every day, but in practice, it should be used only occasionally -- if the Keeper feels a player is exploiting this finanical abstraction, a use of assets may be called for.
So, lets look at someone with a rich credit rating CR, of for example 90:
|CR x 20
|CR x 2,000
They would have $1,800 distributed in cash, maybe $100 on their person, $1,700 in a safe at their mansion. They own assets worth $180,000, for which the player should define what they are - e.g. said mansion. And they can spend up to $250 in a given day, without needing to reduce their cash or sell assets.
So, if they wanted to hire a crew of 4 laborers for an expedition to help you dig out an ancient site, checking the monthly salary for a laborer from the sources above, that would cost you about $200 per week, or about $30 a day, and would be well within the spending level. Unless they also spend a large amount in a day on other stuff, probably not worth worrying about. After all, that is the fun of being rich.
However full blown expedition with a professor of archeology, drivers, a larger crew, ex-military for protection etc., that costs close to 250 every day probably would not be OK, and should be funded separately, at the keepers discretion.
Technically, if the character pays for example a manservant on an ongoing basis, you could deduct such running costs per diem from the spending level, to arrive at a new, lower spending level of how much they can spend in a given day without needing to tap into cash. I would not worry about this unless it was starting to be a significant share of the spending level though.
The rules give you judgment as a Keeper on how to handle these things. Looking to exactly quantify and track spending within Spending Level is really not what the rule is intended for.