N.T.
I think you handled this well - very generous to offer to buy such a large item! I have a DaVinci convertible crib that we bought from amazon.com for about $250 w/ free shipping. We will likely use it as a toddler bed, but as with most convertible cribs, when converted it takes a full sized mattress. While a full is not much bigger than a twin, my little girl's room is not particularly huge. But the bed is beautiful, and I can always use it should I have a second child.
Anyway, amazon.com offers free shipping on many cribs, and has reviews to help your fiance's daughter make an informed decision.
Good luck!