Supposedly it tops the High Tea at the Park Hyatt just 2 1/2 blocks down Alvear, although that's supposed to be a nice one too. And if you can have it outside vs. inside (I don't know if this is possible), then on a nice day you can't get better than that.
My 2 friends went there for brunch a couple of months ago - it set them back $150.00 including tips (dollars). But I think they went on a Sunday, if that makes a difference. Foods great; she'll be impressed big time.
I went the other day and paid 120 pesos per person. It's a HUGE serving so unless your very hungry I suggest sharing with your partner. If you share, you pay an extra 15 pesos for the second tea.... all up 135 pesos.
P.S. You need to ask for whipped cream with the scones otherwise they just give you butter