Just double the prices and round down to the nearest 0.1 $

So for example:

Market price for product is 3.21 $
Double it to 6.42 $
Set the price to 6.40 $


2nd example:

Market price for product is 4.50 $
Double it to 9.00 $


You could set the prices to 2x market price without rounding, but then you have to handle prices with 0.01 and 0.05.



Have fun :)