If I may offer a perspective different than the first couple responses, you probably need to start by asking: is this primarily a business or a learning experience?
This is not trivial; if it is a business, you've clearly portrayed yourself as the boss, and you've asked how to price her labor. If it is a learning experience for her - this can evolve into a "business" - the agency probably needs to rest with her.
If it doesn't matter to you whether she gives her pens away for free or makes a healthy profit, why not empower her with the tools and information to decide? How might her decision process change if she were paying for the supplies that you currently purchase? Of course, allowing her to cultivate a sense of generosity and pride in her work is certainly worth something, even if completely outside of the financial realm, but she must appreciate the "true costs" of the endeavor. Just as it may not be "fair" to you that she sells pens at a price that might not even cover your costs, it's not "fair" to her development as a young entrepreneur to limit her ability to make decisions. That sounds like a recipe for resentment and a total motivational killer.
If she's your employee, I think you can set her labor prices, but if this is a learning experience, maybe she needs to be in the driver's seat? She might make the ten pens and realize "Wow, this is awesome that I can make gifts for people and make them happy, and I'm more than willing to forego any sense of profit", alternatively, she may think "This is a LOT of work, and I won't make enough profit to do this again".
Given that her opportunity cost is probably very low, there's not much risk for her in this approach, again assuming your intentions are for her to learn her own value.
At the end of the day, she will know her value far better than any of us speculating with our own personal and somewhat arbitrary formulas. If she's the boss and empowered to make these decisions, that rate can change over time, echoing part of what was stated earlier with differential rates.
Just my $0.02.