Rohan has a total of 20 x 2 = <<20*2=40>>40 coconut trees.
And he has a total of 40 x 6 = <<40*6=240>>240 coconuts.
So, the total coconut he will have after 6 months is 240 x 2= <<240*2=480>>480.
Therefore, the total amount he will earn is 480 x $0.50 = $<<480*0.5=240>>240.
#### 240