Economic and practical arguments are good. But don't forget safety - swapping aircraft regularly will increase your chance of getting it wrong down low. You need your own ship to get intimately familiar with its flying behaviour. There are probably some other irrefutable safety benefits if you think hard. Knowledge of parachute history and aircraft maintenance?
Of course play this card carefully and know your audience - for instance if you spent 6 months convincing her that gliding is not dangerous then maybe disregard this argument line!

Cheers Ben