Rounding off the currency conversion in a Drupal Commerce website

I have a Drupal website (a catalogue one, without e-commerce functions) with a currency conversion from US Dollars (original currency: US$) to Argentine Dollars (target currency AR$). The conversion is managed by the Multicurrency module, a submodule of DrupalCommerce. I need to round off the price resulting by this conversion by 50 Argentine Dollars. Example: the product X is 100 US$; the conversion rate is 1 US$ = 17.80 AR$ ; so the conversion for the product X is AR$ 1780; I need that it changes to AR$ 1800. It could be done by two different approaches: a) hacking a module (the multicurrency itself or a submodule like [login to view URL]) b) using the Rules module. Of course I would prefer the second approach, if it is possible.