It seems like one of the themes of the book is how unfair the feudal system was and how the nobility were able to take advantage of the lower classes. It looks like here that Twain equates the pay disparity in his own day between architects & engineers and manual laborers (well, it is the time traveling Hank Martin who says this) with the rigged system he saw in favor of the upper classes in Arthur's time.
But the pay may need to be higher for architects, engineers and musicians, for example, because they have to go through long and expensive training. Also, the supply of unskilled labor might be very large, which will tend to hold their wages down. It is not necessarily the result of any exploitation or oppression or inherent unfairness in the system.
One other interesting observation he makes is that doing intellectual work is its own reward and Twain says he would not mind getting paid only a little to do such pleasurable work instead of getting paid much more to do manual labor. There are always trade offs and it is not surprising that he would give up pay for better working conditions, which can include how much you like what you are doing.
Anyway, here is the passage from chapter 28.
"There are wise people who talk ever so knowingly and complacently about "the working classes," and satisfy themselves that a day's hard intellectual work is very much harder than a day's hard manual toil, and is righteously entitled to much bigger pay. Why, they really think that, you know, because they know all about the one, but haven't tried the other. But I know all about both; and so far as I am concerned, there isn't money enough in the universe to hire me to swing a pickaxe thirty days, but I will do the hardest kind of intellectual work for just as near nothing as you can cipher it down--and I will be satisfied, too.
Intellectual "work" is misnamed; it is a pleasure, a dissipation, and is its own highest reward. The poorest paid architect, engineer, general, author, sculptor, painter, lecturer, advocate, legislator, actor, preacher, singer is constructively in heaven when he is at work; and as for the musician with the fiddle-bow in his hand who sits in the midst of a great orchestra with the ebbing and flowing tides of divine sound washing over him--why, certainly, he is at work, if you wish to call it that, but lord, it's a sarcasm just the same. The law of work does seem utterly unfair--but there it is, and nothing can change it: the higher the pay in enjoyment the worker gets out of it, the higher shall be his pay in cash, also. And it's also the very law of those transparent swindles, transmissible nobility and kingship."