Only Sigma currently make lower cost telephoto alternatives to the major manufacturers and even the Sigma lenses aren't exactly cheap.
Of course there is a reason why such lenses aren't cheap. Big lenses need large elements and telephoto lenses need exotic glass, neither of which are easy to make or low cost items. However I agree the major lens makers have priced telephotos out of the reach of the vast majority of non-professional photographers (and even some professionals). Today's lenses are loaded with features like multiple AF modes and image stabilization, which all add to the cost.
It's probably not possible to manufacture an optically excellent 400/2.8, 500/4 or 600/4 at what most people would regard as a "reasonable" cost. You might
be able to do a 300/2.8 or 400/4 at around $2500-$3000 if you cut down on the frills. In fact the very good Sigma 120-300/2.8 EX DG IF HSM APO
is "only" $2999.
You could also probably do a slowish 500/5.6 for around $3000, though nobody seems to think that such a lens would sell well.
There are some fairly cheap telephoto lenses, but they aren't particularly good, fast or convenient. Vivitar/Bower/Optika/etc
have a 650-1300mm telephoto for $250
. The downside is that it's f8 to f16, manual focus. I don't know how good it is, but it's a 8 element 5 group multicoated design so it's probably worth the $250 you pay for it. There are even cheap mirror lenses.
Probably the best bet today for a modern telephoto lens with pretty decent performance and all the bells and whistles is the Sigma 50-500mm f/4.5-6.3 APO DG OS HSM
at around $1500 which I've reviewed and which I was pleasantly surprised by.
The Canon 400/5.6L USM
is also a relative bargain at around $1300 for an excellent telephoto lens of modest aperture. As long as you don't demand IS it's probably the best value in any current high quality telephoto lens. Beware of wishing for a MkII version with IS though as I suspect that would push the price onto the wrong side of $2000.