I'm looking for, as the title suggests, a cheap but fun online rts to play with my friends. (Cheap implies my friends won't buy SC2 even though it is the best PC game EVER) So any suggestions would be appreciated.

TA: Spring is one of the better ones around, basically Supreme Commander (it is actually based on Total Annihilation hench 'TA') but with more basic graphics, however it is a much more slick experience. It has however recently been made a lot more complicated in setting it up as you need to download the spring engine (almost like a client) then  a multitude of other things  to get it working, it used to just be a single .exe, but once it is up its smooth sailing from there and it has a very robust online component.

Homeworld 2 is just about the best real time space rts of all time.

Sword of the stars, probably the very best turn based strategy of all, and that includes games such as Galactic/Civilisation and Space Empire.

I could suggest others but to be honest you can't go wrong with those three. The good thing is that in a week or so the steam post christmas/january sales should make many games stupidly cheap.

Warriors Kings Battles!!!!!!!

Better than SC2, without a doubt.

I'll check it out, thanks

Thanks. I'm downloading TA Springs right now to check it out

Homeworld 2 is like 30 € so thats to expensive for me, sorry

And tbs isn't my thing, so I dont think Sword of the Stars is gonna be for me, but ill check it out anyway.

EDIT: Can you send me a PM of exactly what you need to get TA: Springs, because i cannot find all of it

Age of Mythology

The Battle for Middle Earth. (I is better than II)

warcraft 3 is still fun and some of the custom maps are a tone of fun as well, you might also want to look into the warhammer 40K games 

Tribal wars 

Free and thousands of players at once, online friends friendly (if you end up in the same place) and its pretty crazy but at the same time it can be either extremely boring to some but extremely addicting. There is no middle ground tho. Its an Browser game so no graphics card required because this is an Text based game. Thats right there is 190,000 (used to be 800,000 2 years back) all playing an text based game.