AIBU asking for payment for a job?

[TFan]

My youngest daughter (16) is a very talented artist. She recently posted some sketch pics to Facebook and my mum saw them and asked her to do a large picture for her. In the message to my daughter she did say she'd pay her for doing it. She wanted a large 24" x 24" picture doing of a specific superhero, this involved my daughter buying materials as she didn't have a big enough canvas or all the required colours of alcohol markers etc the initial outlay was £30
It took her 30 hours in total to complete in between finishing her exams, applying for jobs and socialising etc
We dropped the painting off last week and my mum and her partner were thrilled with it. It's framed in pride of place in the lounge. We stayed for an hour and no mention of paying for the picture came up.
We left and I told my daughter I'd mention it to my mum if she'd not paid over the next day or two. The next morning my mum posted a card with 30 quid in it so that just covered costs really.
Eventually I got the courage to mention to my mum that £30 only just covered costs and she messaged back saying "So you want me to give her more money or what?"
I've not replied yet. I know my daughter should have sorted rough pricing with her at the time but she's a kid and a bit anxious about things like that. And I know my mum didn't know how much the materials would cost (although I have mentioned it in passing to her while the picture was still being done) I feel terrible now that I've upset my mum and my daughter did 30+ hours of work for nothing.
Was I unreasonable to point out that £30 only just covered costs? But if someone does a commission piece for you, would you just presume how much you should pay for it or would you ask?

I'd be mortified if my parent offered to pay my DD for something and then reneged on it.

Yes, saying "I'll pay you for it" is very different from "I'll reimburse you for your material costs" - which should be a given anyway, and in no way a 'generous offer'.

The reality is that people don’t ‘get’ how much creative crafts cost, either in terms or time or materials.

Absolutely right, they don't. In fact, in the case of an original painting, most people will judge its value by how nice or fancy the frame is, with all the artist's hard work and 'unseen' materials effectively treated as worthless.

See how many people will ask to buy the work unframed and then react genuinely shocked to learn that omitting the cost of a frame only reduces the price by a very small proportion.

I think the suggestion of giving it as a Christmas present is a good one - whether from the granddaughter or from OP, having paid her daughter for it.

That way, the gran isn't expected to find any more money, if she's struggling (or just tight), but it then makes it explicitly clear that the main value of the piece is not just the cost of the materials.

I definitely think if she said it was to raise money for college, put it back in her boat. It's up to you, mum. Daughter was hoping to raise money for college, £30 covered the cost of materials I'm sure dd would appreciate any more you'd like to offer.

Also presumably she still has the pens, so she can use them for other commissions?

I'd put it back on her. Just respond 'well she has just covered costs so has done 30 hours of work for you for free. What do you think would be fair? '