Nothing in life is free my friend.
However, if you do plan on spending a couple of bucks on RTS games I can give you a couple of hints.
First things first, download STEAM [http://store.steampowered.com/] and install it.
From there you can purchase a big number of RTS games fairly cheap.
Beware though, some of the prices are a little bit off (twice the price) compared to retail pricing so you better be a tad careful and compare the prices with your local games shop before buying anything.
Right now you can buy Civilization III on STEAM for ?5 which to me sounds like a good deal.
While not anywhere near the same style my big personal favourite is Company of Heroes (?10).
Also, not available on STEAM, the old Age of Empires games are quite similar to Civilization. I still play Age of Empires II online with friends and it's a lot of fun. That game shouldn't cost you more than 5 bucks.