It largely depends on the adopt. The more effort put into the presentation typically relates to how much it's initially worth. Here's a few factors I've noticed

1) Reusing lines lowers the price of the adopt as its percieved worth drops since it was done in a batch (And vice versa)
2) Adding shading and/or other stuff off the bat increases the price of adopts
3) Your strategy of making batches of free adopts and charging for evolutions seems to be a good way to attract buyers, even if not everyone will pay for upgrades

As for appealing upgrades
1) Lower priced ref sheets. Not everyone wants Gigantamax and/or megas, so a lower tier of ref sheet could be profitable
2) Shading upgrade per form
3) Tera form? Pokeball? Idk
4) Outfit alt

As for species, the main popular lines I've noted are
- Buisel
- Eevee
- Shinx
- Riolu
- Pretty much every starter
- Absol (as a one-off)
- Zorua
- Bidoof for some reason
- The newest pokemon of any period
- Legendaries/Mythicals
- Gible
- Rockruff
- Buneary
- Ralts
- Vulpix
- Pokemon fusions

Good luck!

Eevee

Yumper