For a long time I was able to keep my properties stocked with hotel toiletries I gathered while traveling, but when I had more places I went with Accent Amenities (their website is accentamenities.com) which does cases of 100 each of little soaps, shampoos, conditioners, lotion at around 27 cents each. They will send you a sample pack for $5 that shows all their different lines- I really appreciated the chance to see, smell and use the various types.
I ended up going with one of their lines (called Nurture) that looks fairly high end but is pretty affordable- a full set of 2 soaps (one for the sink and one for the shower), shampoo, conditioner, and lotion, runs around $1.30 per bathroom, plus I supply toothbrush, toothpaste, shaving cream and razors, (also from Accent, average 40 cents each) but these rarely get used. I am now starting to provide big bottles of shampoo and body wash in the showers in addition, and some people seem to use those and not the individual toiletries that are provided.
All in all, I'd say only about half the toiletries I put out get used, so I don't spend that much. I also have a little basket in each bathroom with sewing kits, emery boards, shower caps (all of these I get from my travels) plus I bought a big box of "La Fresh travel-lite" individually wrapped makeup remover towelettes online (which I highly recommend- I have lost fewer towels and facecloths to makeup once I started providing these), etc.
I provide snacks for people when they arrive, e.g. a couple of granola bars and a couple of bags of salty snacks like pretzels- I go to Costco and see what they have that's individually packaged, has an extended sell-by date (some of the chips have to get used up quicker than I would use them) and then look for something that runs less that 45 cents each.
In high season, when my cottages are renting for $200+ a night, I provide a bottle of wine at checkin as part of the basket. I don't spend more than about $8 a bottle, and have gotten some nice $6-7 a bottle whites recently. I would like to stop providing it as low season is starting for me here in Florida, but several of my five star reviews have mentioned it specifically so I will probably just keep providing it.
I know I overdo it in terms of amenities but people do seem to appreciate it, they mention it a lot in the reviews, which are 93% 5 star on AirBnB and I guess in the end I want to offer a super nice experience- I can't help wanting people to feel pampered!
Good luck- W