How can I convince my parents to let me buy a new lawnmower?

I am 14 and cut lawns to make some extra cash. The thing is i use my parents 25 year old lawnboy (that miraculously works perfectly even being so old) the mower isn't self propeled so after one lawn I normally call it a day. So right now it's tiring and I can only cut one lawn per day. I can get used self propeled mowers on craigslist but my dads always like, "you can use our mower" any ideas on how I can convince them to let me buy a new one to make me more efficient?