You can buy cheap tickets with Pobeda on pobeda.aero, ideally at the start of the sale. That's when tickets are cheapest and prices increase as you get closer to departure. However, this is only true for international flights, while for domestic flights within Russia it is not worth buying too far in advance, but perhaps just a week to 14 days before departure.
Pobeda flies from Bratislava to Moscow five times a week. Prices for return tickets range between €100-150. It is also newly offering twice-weekly flights from Karlovy Vary from February 2018 at similar prices to Bratislava. North-Czechs can also take advantage of flights from Leipzig or Berlin.
Please note that a 2% card payment fee will be added to the final price.