What does the trip involve? Is it the entire unit going, or has she been selected for a place specifically?
For a unit trip, I don't think she'll manage to fundraiser anywhere near that amount. For a trip she's been selected for, as long as the unit are supportive, it's probably doable. If it's a unit trip, I don't think they'll be expecting girls to fundraise much independently towards that total, but there may be some group fundraising to bring the total down (either overall, or for those who participate). What info have you been given already on payments?
Have a chat with the leaders and find out about any payment deadlines, support available, unit fundraising, and what others are doing or have done in the past. I'd say if it's going to be too much of a stretch for you to pay for it, say no, as there's no guarantees with fundraising, particularly for a unit trip where there'll be lots of other kids in the same boat (so no grants etc). Maybe chat to the leaders about whether there's other trips likely too, as they're likely to be a really keen unit if they're going international trip, so there may be other residentials coming up at a lower price point.
I'd also bear in mind if your daughter hasn't been attending long, how committed is she, as a trip that expensive is likely to be being planned quite far in advance, so check she's committed to being actively in the unit that long, outline the requirements and committment needed etc, as you make it sound like she's only been attending for a short period of time.