I just went away and did a calculation of what my sister gave me when I took my niece with us to DLRP last November - it was £100 for a 2 night/2 day trip. We spent it all (and that was with breakfast included each morning at the hotel). Niece did get a few souvenirs from that, probably about 50 euros worth (but you have to remember I have a Dream AP, so all our meals were 10% off, and all her souvenirs were 20% off). But the rest was on food/drinks/popcorn etc for her (and did include a brilliant character lunch at the Lucky Nugget).
So you could be doing with £150 or £200 for your niece couldn't you?
Elaine