For spot-finetuning with Dreambooth (not as good as full-finetuning but can get a specific subject/style much faster), it can be done with about $0.08 of GPU compute, although optimizing it is harder.
As it is right now, stable diffusion can generate adult imagery by itself, however it seems like it's been fine-tuned after the fact to try to 'cover up' that fact as much as they could before releasing the model publicly.
As far as textual inversion, JoePenna’s Dreambooth [2] implementation uses Textual Inversion.
[1] https://github.com/CompVis/stable-diffusion/commit/a6e2f3b12... [2] https://github.com/JoePenna/Dreambooth-Stable-Diffusion