Its very undetermined how your game should look.

If its just something that should play somehow like in an RTS, and Kids like it,
you can make this realatively easy.
If you want to make a competative LAN Tournament-RTS game, it will cost much more.
Without a gamedesign draft, thaere is no way of saying how much this will cost (eg how many people
have to work on it for how long - wich ist the major cost in gameproduction)

RTS (especially Multiplayer) have a huge portion of Coding required to make it work. Much more than the Casual games
you see on the Indy sites, and more than an Ego-shooters. For this you need skilled and experienced programmers,
and they cost of course.

Actually, if you have to ask in such a way, you should not attempt a big production in the first place.
Better you start by joining a group of experienced developers, or look for employment in a game-company, to learn about
this risky market.

If you need some numbers, I can quickly think up some:

Take http://www.arenawars.net/ for example, a pure Multiplayer-Tournament RTS.

I suppose you need 3 Programmers, with advanced skill, working for two years.
One graphical Artist with lowpoly skill.
At least one person who takes care of Game/Leveldesign and Balancing (who needs to be
a good competative RTS player, and has experience in what makes RTS games fun)

Additional parttime work by Sounddesigners, additional Artits.

A Person who takes care of project planning, marketing, financing, Public -relations, (Producer)

If you take a 6 Man team, working for 2 years, each earning 3000$ per month (wich is low),
You will have 432000$ cost, just for the employees.
Lets say 100.000$ for licencees, equipment and rent,
200.000$ for marketing and production (low number).

you End up with 732.000 $.

I really took low numbers...