My minimum charge is $25 and those lawns all take me less than 25 minutes to do solo, typically between 15 and 20 minutes. If I were going to be somewhere for 50 minutes, I would want around $50. The best way to increase price is not in the middle of the season. You will want to raise your prices at the beginning of the season.
What you really NEED to figure out is how much it costs to run your equipment per hour. That will let you figure out how much you need to charge per hour to make money and still cover all of your expenses.
'07 Chevy 2500HD
2010 John Deere Z925A 54" MOD
2011 John Deere Quik Trak 647A
John Deere walkbehind