The 70-200 from Sigma had an okay quality, but that was pretty much it. It was okay. From what I read on the internet Canon's version might be better built and reliable, but image quality wise it's not going to be much better. (And their IS version is on the outer edge of my budget)
Second the comment that if you're talking about reviews of Canon's 70-200mm f/2.8L IS II, the internet is lying to you (shocking, I know). Find some honest reviews. The 70-200mm II is probably the sharpest zoom lens in existence today - sharper than most primes in the same focal range. But you certainly pay for that sharpness. Now, if you're talking about the 70-200mm f/2.8L IS (the original/MkI version), then yes, that was ok, but is the least sharp of Canon's five 70-200mm zooms.
The only other affordable lenses I could think of would be the 300/4 or 400/5.6 from Canon. (Plus maybe the 1.4 converter)
Note that while the 300mm f/4L IS works fine with the 1.4x TC, when you put a 1.4x TC behind the 400mm f/5.6 you end up at f/8 meaning no autofocus (except with the center AF point of a 1-series body).
So, any recommendations, experiences or great ideas on how I can get long range at an affordable price? (With affordable meaning less than Canon's 70-200 2.8 IS)
While I agree with the recommendations of the 70-200mm f/4L IS as an excellent lens, I wouldn't call 200mm 'long range' especially since you have a 150mm prime.
How much less than the 70-200mm f/2.8L IS? And do you mean less than the MkII version of that lens (US$2500), or the older MkI version (harder to find, but usually ~US$1800)?
Will your budget cover $1500-1600? I'm guessing yes, since the 300/4 + 1.4x II will run you about that cost. If so, I'd recommend either the 100-400mm f/4.5-5.6L IS or the 70-300mm f/4-5.6L IS. The new 70-300mm L zoom is sharper and has better IS and weather sealing, it's also smaller and lighter. If 300mm is sufficient for your needs, that's the way to go.
But, you mentioned the Sigma 70-200/2.8 + 2x TC, so you may need 400mm. If that's the case, I'd really recommend the 100-400mm. Yes, it's an old design. But so what - it delivers great image quality. That's the lens I use 95% of the time with my 7D. Sample pics? Here are several:
EOS 7D, EF 100-400mm f/4.5-5.6L IS USM @ 400mm, 1/640 s, f/5.6, ISO 125
EOS 7D, EF 100-400mm f/4.5-5.6L IS USM @ 400mm, 1/1600 s, f/6.3, ISO 1600
EOS 7D, EF 100-400mm f/4.5-5.6L IS USM @ 400mm, 1/640 s, f/5.6, ISO 200
EOS 7D, EF 100-400mm f/4.5-5.6L IS USM @ 400mm, 1/640 s, f/5.6, ISO 100
EOS 7D, EF 100-400mm f/4.5-5.6L IS USM @ 400mm, 1/1000 s, f/6.3, ISO 3200
Here's a 100% crop of the last one to show the detail: