Normally, I am the first person in line to insist on charging a liveable wage for quilting. It drives me nuts when people undercharge for their work, but ... this isn't some neighbor or distant cousin, it's your mom. I don't know what kind of relationship you have with her or how long she's been with the boyfriend, but assuming he isn't just some guy that dropped into her live within the last few months, I would probably charge my mom for 100% of the fabric, thread, rotary blades, needles, batting and such plus probably add on $60-75. I would not feel terrible about giving it to her just the cost of materials, but it sounds like she maybe wants to commission a quilt for him. Part of the amount I listed goes to cover electricity, water, wear-and-tear on durable supplies like cutting mat & sewing machine and the rest is a super modest charge for my time. I'd be very clear to tell her that it's clear that this man makes her very happy (assuming that's true) and that since I love her I want to give her the special one-time "Just for Mom discount".
If she absolutely insists on paying full price for my materials and time, I think $350-400 (including materials) is probably appropriate since you're not only quilting, but also cutting, piecing and binding the quilt. If you are doing anything more than the basics (for example, if you plan to do any applique, design work, embroidery, or custom quilting), I believe the price should reflect that. We all know that making a twin sized quilt from start to finish can easily exceed 35-40. Only allowing yourself $10/hour (what people in my area pay someone to clean their house or take orders at McDonald's) is already devaluing your skill as an artist. I pay $70 for a 60-min basic mani/pedi. I find it hard to swallow when people insist I am only 1/10th as skilled as a nail tech whose work lasts for 2 weeks max while my quilts should last for 20 years. The only struggle I'm having is that it's your mom that's asking. Otherwise, here's what I'd charge:
$20/hour for washing, pressing, cutting & piecing
$0.03 - $0.06/sq in for quilting (less for all over design, more for multiple designs) + $5/spool for thread
$3/linear ft for binding (making binding & attaching binding -- I also include label for this price)
PLUS cost of materials & pattern (from preferred designers only since most designers do not allow the use of their patterns to make quilts for sale)
Yes, that means either I or my vendor end up selling appliqued baby quilts for $700-900 a piece. They're still getting them on the cheap when you figure that at $600 (since materials cost at least $100), that means I have to do the entire quilt in only 60 hours to make $10/hour. Fortunately, I live in an area where people appreciate fine art & understand quality or I'm sure it would be worse. I'm hoping as I get better at hand applique & hopefully maybe get recognized for the quilt I'm planning to enter in QuiltWeek next Spring, Irish Sea, perhaps I can turn them out faster and maybe increase my prices.
Good luck finding an answer to this age-old but impossible question.