Hey, quick question: That age range you specified--why? If she's 18, she has maybe a high school diploma, maybe a GED, maybe a trade if she was smart and trained during school. (Which I wish I'd been. Smart like that, I mean.) If she's 22, she has possibly spent four years slaving away at a B.A. or B.S. and now has exorbitant student loans to pay plus zero job prospects. Seriously, four years gets you a secretary job nowadays.
So, are you going to put her through grad school? Find her a job? Those would be great things to mention as incentive. Moving costs, pssh. She could hitch across the country at that age with her entire life in a suitcase. What you really want to cover is what comes next and how it benefits her, especially as she'll be off her parents' health insurance with no ability to get on yours (will one of you marry her?) and possibly crossing borders to get there ("North America" does still include Canada and Mexico). At least narrow it down to "legal to relocate to us".