Maybe paintings are worth more because the people you sell them to have connections that would allow them to leverage the paintings worth outside the prison, making them more valuable than improvised weapons from sticks and stones and banged up armor.
At some point you need to allow digital RPGs some leeway. Creating a different value system for items inside the prison just because you feel its immersion breaking for paintings to be worth more is just as wonky and simply replaces a very minor "not really a problem" with something similar.
Also why do you care about roleplaying if you dont want your moral decisions to have any impact on your income?
If you want to play the roleplaying card, then it makes even more sense that a ruthless character in a law of the jungle type prison would end up way more wealthy than some compassionate good guy. In an enviorment where people are poor you don't get a golden trophy for being a pleasant guy.
And finally, as I pointed out you can just go rob the keep. No moral issues with sticking it to the magisters. All you need to do is overcome your "problem" with paintings being valuable, which you can just as easily argue works with the lore as agfainst it. The only reason to complain about it is if you really really want to complain about something.
