It depends heavily on what you're doing and how, but since I do a *lot* of 2d art commissions, maybe some useful info here. I will assume that you do all of the writing and coding yourself, to save costs.
As with any kind of estimate, these are tailored to the rough quality and return-of-investment I personally expect. If you want fancier stuff (animation, big-name artists like Crescentia, etc) the cost can easily double or triple, or you can get super shitty art for half the price.
High-quality character sprites with multiple outfits, expressions, etc. are generally going to run $100-200 apiece, more if you want a big name, less if you're willing to compromise on quality or use (frankly, abuse) newer artists that undercharge.
CGs vary based on many factors, both lewd and normal, but you're probably looking at $80-100+ as a baseline estimate for a CG of a single character with few/no variations and a simple BG. If you need really high quality, detailed background, or lots of variations, it can skyrocket into 2-5x that range very quickly.
Background art varies from maybe $50-150 assuming you don't need any characters in it, anything below 50 is probably going to be ass but you can get pretty dang good quality in the 70-100 range.
Music is typically gonna be $100-150 per 30 seconds if you want commercial video game quality, but if you look hard you can find some pretty good stuff in the $30-50 for 2-4 min range. As always, you're almost guaranteed to sacrifice quality/versatility for that price, but you can find some real diamonds too.
A high-quality renpy UI commission apparently costs around 50-100, I haven't done that yet myself but I am looking into it.
You can't really put a price on working 8-12 hours a day for months or years, though the opportunity cost of not spending those hours working at a local grocery store is pretty rough, much less a 'real' job that pays 30+ an hour. Even assuming you're working for around US minimum wage (10-15 an hour iirc?) is around $1000-2000 a month of wasted potential income, $5000+ if compared to a full-time job
You can cut out some of those costs by working in daz3d or similar, though then you have to learn how to do them yourself, get a rig/card/etc that can handle them, and consider the replacement and energy costs of hardware that will burn out much faster doing a million renders... etc.
To put it bluntly, I would not expect to even break even. Especially with factors like the US recession, global financial crisis, and the unfortunate reality of how few people are willing to pay for porn, or wait a year or two to see if you're legit before supporting.
Let me put it this way: if I remember Graphtreon numbers right, I think there are a grand total of about 300 H-game patreons that make more than minimum wage, worldwide, BEFORE accounting for any development costs or wages. Only like 8%? even break 500 a month base income, which again doesn't account for their costs.
I'm a pretty small fish, sitting around 440 a month currently, which is A. about 40% of the US poverty line or 12% of middle class income, and B. I spend all of it on commissions, so I'm actually making about $20 a month (which goes to bills) working 50+ hour weeks.
Fortunately, I have a lot of fun working on this and didn't expect to make bank. Hopefully, I can eventually get to a more stable position, but I absolutely would not recommend going into this expecting a quick buck.
As much as people complain about 'milkers,' a couple of devs that got lucky are completely unrelated to the experience of 98% of devs. Even for relatively-successful devs, 3-4k a month on patreon MIGHT barely break into middle-class income if they don't have to spend too much of it on the game. And the biggest-name ones are usually a team, so that 80k+ a month that Summertime Saga is making is gonna be split between 8? people with unknown proportions.
If you don't explode and start making 1k+ a month in the first month, it's probably going to be a 2+ year journey to get anywhere near meaningful income.
