As @royT suggested the best solution would be letting players to host their own games. although you will end up developing a match-making server, but the costs for that one is much more cheaper than a dedicated game server.
But other than that Option let's do some multiplications. first of all you need to check how much data you need to send per game-state. for example let's say there each player can own at most 100 units in the game, and each one can have a position in a plane with 16k*16k resolution. which will result in roughly 28 bits of data for just position. let's say there are some extended data for these units. for example their type, hitpoint, experience and so on. for these extended data I guess reserving 20 bits is reasonable. so far we have computer 48 bits unit, multiply that by number of units and you'll get to 4800 bits for one play units in a game. for each player there are some other variable you need to send, for example how much money they own, or if they are chatting to other player. I guess assuming 200 bits average is more than enough, which will result in 5000 bits per player, knowing that each game consist of at most 8 players we will need 40Kbits or 5KB for just player data in each game-state. of course every player only needs to send his own data, but needs to receive game-states from all other players, so in total server needs to send 4375Bytes to each player and receive 625B from him. meaning exactly 5KB bandwidth for each player connected to server per tick. multiply that by number of ticks per second and you'll get to the 50KBps and multiply that number of seconds in a month (2592000) and you'll get to ~129GB bandwidth usage per month per player. so your server needs ~13TB bandwidth to handle all the traffic! I know these numbers seems a bit too much crazy but that's what you'll get by the values you provided.
but that's not how big RTS titles were implemented! I'm not sure how they did handle their transactions but they usually use less bandwidth for example:
- WarcraftIII : 3KBps down and 1KBps up.
- BattleForge : 5KBps down and 1KBps up.
- DotA2 : 10KBps down and 3KBps up.
- StrongHold crusader : 5KBps down and 3-4KBps up.
(note that all these numbers are what I can remember from the time I was/am playing those games)
putting these numbers as a base you can see there still is a huge bandwidth usage server needs to handle (maybe around 1TB per month).