Does she have kids who might chip in (with or without her knowing about it) once they know how much she wants it?
You could quietly check with them before quoting her a price.
In fact, I'd tell them, with a smile, "I know exactly what your mom would love for Christmas (or other gifting occasion) 'cuz she TOLD me, so here's a way for all of you to split the cost of something she's already said she'd love to have, plus your shopping would be taken care of -- win/win for everyone" [including you!].