I would say that punishment at school will be enough, you shouldn't need to do additional punishment at home.
I think your daughter needs to fess up, but she's only 10. She's going to need your support more than ever now. I think a closed-door conversation with your daughter, the teacher, the principal and maybe a guidance counselor is a good idea. I think together you can come up with an appropriate consequence. At the very least, the girl she took the test for should get a failing grade and your daughter needs to take the money she made and either give it back or donate it to a worthy cause.
To turn this into a positive, you should get your daughter involved in peer tutoring. She obviously wanted to help, this would be a legit way for her to help others get a better grade. I can see your daughter's side, she thinks like a 10 year old. I'm pretty sure my daughter (who is 10) would have thought it was a good idea too.
Perhaps her "punishment" is tutoring or helping others. Perhaps she can do several hours of after-school help for the younger grades, or she can help the school in some way that's above and beyond.
I have found the most effective "punishments" aren't really punishments but simply redirecting the behavior toward something positive. What you really want to do is teach your child how to channel what they want into a positive outcome.
When my daughter was 6 she tried to steal a toy from the store. I saw her look around and put it in her pocket. I was LIVID, and I made her return it to the store clerk and apologize. I asked the store clerk if she'd mention that they call the police on people who steal to scare her.
But then I went home and turned it into a positive. I made her do extra chores to earn money so she could legitimately BUY the toy. By the time she earned enough money she didn't want that toy anymore, but she did proudly buy herself a different toy. It was more effective than punishment, and to this day she asks for extra chores to earn money instead of thinking about stealing. She was so scared and embarrassed at the store that it was punishment enough, and she didn't want to go back to that store for about a year for fear they'd "recognize her."
Good luck to you!