The vow mostly just impacts the lower ranks and something that involves binding apparitions isn't going to be shared with people who aren't at least trusted magisters.
Mathilde, a trusted Lady Magister with a dedicated income stream from a highly successful trading company and very few expenses because she can wrangle rent-free acommodations she is not responsible for the upkeep of, largely fold her food, laundry, etc. budget into the operating expenses of her current project, and pay for her personal research expenses by having the library she runs get her books for her, has 831 crowns.
To get performance approximating that of the Rider, you're going to need to hire several
very good mercenaries, because benefits like "ignores armor" and "can't be hurt by most nonmagical weapons" require either getting someone quite high on the power scale or making up for them with numbers. Now, a whole mercenary regiment is probably overkill, but we have a cost for hiring one for a month, at 10,000 crowns, so we have a starting point for how much this might cost.
Let's start by slashing that by 90%, on the grounds that it is overkill. That's still more money than Mathilde has, and for the quality of mercenary you need to hire to match up to an Apparition's basic statline, that doesn't seem too unreasonable, but let's cut it in half just for the sake of argument. That's... Within her means, strictly speaking, but only for a single month, and it takes more than half her reserves of cash when she is, again, in a very good position relative to most people, even most non-Grey Wizards.
Mercenaries, particularly high-quality trustworthy mercenaries, are not cheap.
Honest question: can somebody explain the logic behind having cavalry acting as bodyguards to me? The strength of cavalry lies in its mobility and ability to deal charges, not in standing around (or appearing around) to defend a lone individual, so it just seems like a poor use of the available resource to me. I'd be completely down for bodyguard with a humanoid apparition or something, or even a red in a non-cavalry form, but with this form it just doesn't click to me.
Unlike mundane cavalry it doesn't die if you stab it with nonmagical weapons and physically can't be unhorsed. That changes the situations you can use it in significantly.
