1. It’s best to carry US Dollars and change them in the city centre. You will get amazing rates. You may change some money in the airport, for going from the airport to City Centre.
2. Use the uber app to hail a taxi from the airport to the city centre, especially if you are in a group. Uber is by far the cheapest. For example, for a one way ride from Moscow Sheremetyevo Airport to City Centre, Uber costed me 850 Rubles, whereas official taxis quoted 1500 to 2000 rubles.
3. Regarding sim cards and data, just buy any sim with a data package from the airport, and freely use WhatsApp calls to call back home. Don’t bother to subscribe for international roaming, as data local data packages are quite cheap, and you get the sim within 5 minutes, even though your passport will be required for registration.
4. It’s best to walk around the city centre. You will get a chance to soak into the architecture, the ambience of this amazing city.
5. I used booking.com to find the cheapest hostels in the city centre.
6. For public transportation, it’s best to get a Troika card, which can used for all types of public transportation in Moscow. However, if you are in a group, for short hops, an Uber might be a lot cheaper.
7. Your fan I’d entitles you for free public transport on a few specific routes to the stadium, only on match days. Ofcourse with the same fan I’d, you can travel to different cities for free, but you need to reserve your seat in advance from http://www.tickets.transport2018.com
8. Feel free to ask for a discount, wherever you go. I was searching for a boat ride around St Petersberg, by the river. All the sellers quoted 1000 rubles as fixed price. I requested for a discount, and managed to get on that 1.5 hour tour for just 400 rubles.
9. The official FIFA fan shops at the stadium and at the fan parks are way cheaper than the official store in the City Centre, such as the one in Gum Shopping Mall in Red Square, Moscow.
10. For cheap mementos/souvenirs in Moscow, I discovered a small shop in Khrustal’nyy Pereulok, one lane behind St Basil’s Cathedral. It has a small pedestrian walkway, and the lady in-charge is very friendly, and always gave a good discount; but you need to ask for your discount. From cheap FIFA Caps to magnets, you will find everything there, but at half or one-third the price. Here’s the google link https://goo.gl/maps/cQRRx2FDNa42
If you have more queries, please feel free to comment, and I will answer them.