Repairs are generally better made with epoxy resin rather than polyester. Epoxy has much better adhesion characteristics than polyester, it will form a stronger repair bond.
Because of the extent of the damage, the rudder should be removed from the boat and cut open to inspect the rudder post and the flanges. The barnacles suggest the damaged rudder has been in the water a long time. The foam inside and the metal may have begun to deteriorate and need replacing. This video series will give you a good idea of what's inside a rudder and how to repair it.
As for the cost of repair, I can't venture a guess as you are in Latvia and I'm in the US, don't have an idea about labor rates in Riga or the cost of materials.