Because of how excited I got when I received gifts in the past. For example , I cherish an expensive scarf a girlfriend gave me even though I would never have bought it for more than a few dollars myself.
If you also get excited by cheap gifts (flowers, candy, nice thrift-store finds, whatever) then tell her that. If the gifts have to be expensive for you to appreciate them, though, that's kind of awkward though maybe if you give her gifts of similar value it comes out fair.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.