FIFA have announced the ticket pricing the Russia 2018. The cheapest matches for non-residents start at USD$105 for category 3 group match seats an go up to USD$1,100 for the best view in the final. There are category 4 tickets available to Russian residents that start at around $22. The Confederations Cup ticket sales end on 2nd July 2017, so it is expected that World Cup tickets will start becoming available soon after that in phases.
If you're after international football in the World Cup venues, the FIFA Confederations Cup in 2017 tickets start at a more affordable USD$70 and are already on sale on the FIFA website.
Getting Russian Roubles can be a hassle so be sure to sort it in advance, as well as your travel VISA which you will likely need to get into the country.