I used to work on the bottom rung of the comics industry.
Much of what I did was helping other artists - I did a lot of background coloring under the direction of another artist, who told me what he/she wanted and I delivered. Payment could be by the hour or by the page. Hey, it’s one way to break into the industry, get a feel for the work, and so on.
Mind you, the artist(s) in question were paying me out of their own pocket, usually because they were behind on a deadline and paying me to help them was cheaper than taking a late fee penalty.
The reason you don’t get just one figure for payment for a task is because it varied all over. Your pay depended on who you worked for (DC or Marvel paid more than, say, First Comics of Chicago), what you did (as pointed out, interior inkers got more than interior colorists, and letterers were another group entirely), how experienced you were (the better known/established artists do get paid more). An established artist doing work for a smaller company (perhaps to get their own ideas published) might well be paid more than a less known artist doing similar work for a big company. How well you yourself can negotiate also had a profound effect on pay scale, especially when you leave the land of flat fees and enter the realm of royalties.
And yes, lettering a book IS harder than it looks. Sizing the letters to the space, and being absolutely consistent in your font, are just two of the many factors involved.
Another thing to keep in mind is that the numbers quoted are almost always gross wages - when I was involved in this (oh, gosh, over 15 years ago now!) artists were independent contractors. That meant the comic company handed you a check and YOU got to pay all the taxes out of it. Also, any business expenses, such as studio rent, supplies, shipping materials (When we worked for Marvel the work would need to be FedEx to New York) and yes, additional help to meet those deadlines came out of YOUR pocket. No benefits - no paid vacation, no health insurance, no 401(k) plan, nada. Most of 'em hired an accountant at tax time, and also used a lawyer to go over contract details, which were additional expenses but if you didn’t pay for them you had a very good chance of getting screwed by either the employer or the IRS.
I enjoyed the work, but at the end of the day I had worked really hard for not very much at all, dollar-wise.
I’m guessing they stopped published wage numbers because every wet-behind-the-ears newcomer was demanding at least average wage. I’m sorry, it doesn’t work that way. If an artist fails to meet a deadline, or does substandard work that another artist must fix, it can get really expensive for the publisher, they’re taking a risk, too. You start with low pay and spend time making your deadlines and doing good, reliable work - THEN you can ask for more money. Trust and reputation can be extremely important. I knew someone who was a fantastic artist, but as soon as deadlines started being missed people stopped hiring. One company actually refused to pay, and when the complaint arrived threatened to take the artist to court for breach of contract. Very, very ugly. Let’s be honest - I worked for that person at one point and stopped doing so when I wasn’t getting paid regularly, the erratic behavior got totally out of hand. I then picked up some work when the artist hired to finish the project hired me to help maintain some continuity in the artwork but there’s only so much you can do. Some of the book’s fans noticed and complained (they also noticed it came out late) but what can you do? Their favorite artist was fired, plain and simple, and was never going to be hired by that company ever again. That person’s failure meant the publisher had to get what would normally be 6 weeks of work done in two, which meant hiring bunches of warm bodies to work 12 hour days (no, we didn’t get overtime - we’re independent contractors, they don’t get overtime). Thi$ wa$ expen$ive. When the dust settled that one issue cost four times what it should have to get it out the door.
Anyhow, like I said - I mostly enjoyed the experience but I eventually moved on to other things. There are easier ways to make a living, and quite a few of them pay more, too.