I didn't take my wife out to the field until after I had soloed because I didn't want the additional distraction while I was learning. When I did take her, she had a great time but told me afterwards that she was so afraid I would crash that she didn't want to go again It's the same reason I don't go watch her jump her horse.
As far as the number of planes, it all depends on what you find interesting about the hobby. One guy in our club has been flying the same Big Stik 40 for over 10 years. That is his only plane. One guy has several hundred planes. I am not exaggerating!! He flies a different plane every week. To each his own.
As far as the cost, just tell her what my grandmother told me. "The only difference between men and boys is the price of their toys"
Best of luck
Rufcut