the monetary value of art
The value of art is a function of:
1. What people are willing to pay for the art.
2. The cost for which you are willing to create the art.
It doesn’t matter if someone thinks $25 is too much for a colored sketch, if more people are willing to pay than you can handle, you should definitely charge more. That’s how supply and demand work. Cost complaints only matter if people aren’t buying your art.
On the flip side if you charge “industry standard” prices for personal commissions on tumblr, you might not get as much business as you want. That’s because the people that you’re asking for business are mostly 14-year-olds with no disposable income. They aren’t making a professional, widely-distributed industry-standard comic that will be sold elsewhere. In the industry, asking someone to draw art is an investment that will hopefully pay off in sales. “The industry” isn’t a long line of 14-year-olds waiting to give you hundred-dollar-bills in exchange for a sketch of their OCs.
Value your art. Charge as much as you can while maintaining the amount of commissions you want. If that means you can reach (or even surpass!) industry prices, awesome! On the other hand, if people only buy when you charge $5 per sketch, you are not undervaluing your art! You just gotta become more well known and/or improve your technique.