The fixed costs don't scale down, so your instinct about the $9,000 note is sound arithmetic. Let me put names and rough sizes on the costs so you can set your own floor.
Per note, whatever the price, you're generally looking at a servicer boarding or transfer fee, often somewhere in the low hundreds, ongoing servicing of roughly $20 to $35 a month per loan, a broker price opinion at $100 to $200 if you order one, recording the assignment of mortgage at your county's fee schedule, which differs by state and can run from tens of dollars to a few hundred where transfer taxes apply, and some legal review. Call it around $800 to $1,500 all in for a single small note bought carefully. Against a $9,000 purchase that's brutal. Against a $45,000 purchase it's a couple of points, which is survivable.
That's why the commonly discussed floor for a single performing first lien sits in the tens of thousands rather than the single-digit thousands. Small balance notes do exist, mostly on lower-priced properties and mobile homes with land, and buyers of those tend to buy several at once so the diligence and servicing setup is amortized across a batch.
The route people with smaller allocations actually use is buying a fractional interest, where several investors each hold a recorded share of one note, or investing in a fund that owns a pool. Both give you diversification you can't buy alone. Both also introduce a question your solo purchase doesn't have, which is what happens when the other holders disagree about whether to foreclose or modify. Fractional arrangements can also be treated as securities depending on how they're structured and marketed, which is a question for a securities attorney before you write a check rather than after.
