1. When is it time for you to purchase a professional model bass clarinet (low C range)?
2. What is the cost of one of these instruments (quality of course) and which brands are the best?
3. Is this worth purchasing if you don't plan on being a professional musician?
In terms of #1, I got my first horn when I graduated from High School, right before I went off to Eastman for college. I did all of my auditions on an old plastic Leblanc Vito, low Eb bass -- so I guess that's to say that it can be done. I'd say to get one if you can afford it. You can ALWAYS get rid of a used bass for damn near as much as you paid for it. The market for good, used, professional-quality horns is very tight. Most people who buy them keep them, so if you do need to get rid of one, you can usually find a buyer without too much trouble.
As for #2, you can get a Selmer or a Buffet. Those really are your two options. I'm not a fan of the Yamaha horns because they just don't have the sound of the other two. I'm a Selmer guy. I always have been. Their horns simply sound better, they have a more pleasing resistance (read: they're much less resistant), and they are much more "even-blowing" in terms of some notes being stuffy and others being less stuffy. Buffets are particularly prone to this problem.
Lately I've seen younger students getting low C horns (because you can buy student-model low C horns now - that's something they didn't have in the 1980s), but those student model horns by Jupiter, Allora, etc are total crap. Why? Mainly it's all in the keywork. The keys are really low-quality, they bend and go out of adjustment because the metal used is low-quality.
So, in short, save your money. Don't buy a low-C bass clarinet from one of these guys. You'll have repair bills that, over the course of owning the horn, will probably make up the (admittedly huge) difference in cost.
#3: That's a hard one to answer. I own a great set of pots & pans - probably some of the best - but I'm not a professional chef. Why? Because I like working with good tools. Then again, I cook all the time, so I get good use out of them.
Is this what you plan to do on your bass? Will you use and enjoy it a long time? Does having a good "tool" matter in this case? Probably. If you see yourself not playing for long, or -- and be honest with yourself -- if you are someone who loses interest when things get frustrating…well maybe not.
But then again, as I said in #1, you can always sell it.
Hope this helps some of you guys.