First, which 70-200/2.8 do you have? The non-IS? The IS Mark I? Or the IS Mark II?
Only the latter one works very well with a 2X teleconverter and that's largely just with the Canon 2X Mark III TC. Some people find the image quality of that particular lens with that particular 2X to be acceptible. Others don't.
The two other 70-200/2.8 models really don't work particularly well with 2X.
All can be used with a quality 1.4X to get pretty good results, but that only gets you to an effective 280mm f4 lens.
Adding a TC is the cheapest way, but you generally have to accept slower focus and, in most cases, serious image quality loss adding a 2X to your current lens.
The other "cheapest" options are...
Canon 70-300 IS USM, currently $650 US
Tamron 70-300 VC USD, currently $450 US
Canon 70-300 DO IS USM, very compact, but about $1300 US
Canon 70-300 L IS USM, $1400 US
But to be honest, 300mm usually isn't enough reach for small wildlife/birds. And none of the above can be used with teleconverters. So here are some longer lens options...
Sigma 120-400 OS HSM, currently $900 US.
Sigma 150-500 OS HSM, currently $1050 US
Sigma 50-500 OS HSM, about $1500 US (nicknamed "The Bigma", might want a tripod or at least a monopod)
Canon 100-400 IS USM, about $1500 US (the only "one-touch" zoom in this bunch... some folks really like a one-touch zoom, others don't)
Or prime lenses...
Canon 400/5.6L USM, about $1220 (no IS)
Canon 300/4L IS USM and quality 1.4X teleconverter, about $1600 US.
Pricier, but very versatile, though due to it's size and weight it might need to be used on a monopod or tripod most of the time...
Sigma 120-300/2.8 OS HSM plus 1.4X & 2X teleconverters, about $4000 US
Everything else is going to be more expensive.... sometimes a whole lot more expensive.
If you shop used, you'll find older versions of the Sigma lenses without OS (their version of stabilization), however on such long focal lengths stabilization is very, very helpful. Sigma's OS and Tamron's VC are their equivalents to Canon's IS.
There are some less expensive non USM lenses, too. USM or ultrasonic focus drive motor is the fastest and most accurate type of focus drive on Canon lenses. It's often necessary for wildlife/birds. Tamron's USD and Sigma's HSM are their versions of USM.
I have the 70-200 non-IS version, saldly, I don't think its good with a tc x2 right?
Should probably mentioned that before sorry.
Well as you said 300mm isn't enough, and am pretty sure about that. I've tried 300mm and I know it.
I'll probably skip the "cheapest" option, because it will not make such a difference from my 70-200, an extra 100mm for anew telephoto? Not worth it.
The longer ones is really what am looking for, the canon 100-400mm seems very promising, specially with IS, I heard a lot of people talk good about this lens.
And am not very sure about the sigmas path, I always liked the white boldness of the canon telesphotos, and of course the superior image quality. Is buying the sigma a good way to go? Am on of these people who like to stay in the canon line, never really looked into other lens manufactures.
And for the primes.. A bit uncomfortable, since I would be shooting a variety of Differnt things, getting a prime is a bit limiting, am not sure.
I realized that most of them are f/4, 5.6, why am I not seeing the 2.8s here xD
No need to tell me why xD