Yes, you can make good profit, but in order to do so, you need to be very good at a few things:
1) First, you need to
know the bonsai market exceptionally well: who are the top buyers, where they buy their trees, how much was paid for what tree, etc. Go to bonsai auctions and see what kind of trees are sold, for exactly how much. So, just like in any other business, you need to know the market. The bonsai market is small, but trees DO change hands for money. The information about these transactions is not readily available, you need to know the insiders.
2) You should be able to
recognize great deals, and take andvantage of them. Your profit is a function of what you pay for your investment. If you invest in a tree that you estimate to be worth $5,000 (based on existing sales of trees of similar caliber), and the seller is asking for a fraction of that price (he may be sick, he may need the money right away, he may have inherited the tree and does not care about selling at full value, etc), then you can make a nice profit by selling the tree at full value.
3) (This is related to #1) You need to
know where to sell your tree. If you are trying to sell a high-end tree at a flee market (swap-meet), you will never get what you want. Instead, you need to have a list of potential buyers, and find a way to market to these people.
4) You need to
be highly skilled in creating and maintaining bonsai. This way, you can maintain your investment in top condition, and you cann add lots of value to unfinished material. If you don't have that skill, and you have to pay top professionals to do all the work for you, then you will spend your profit on these people and end up losing money. The profit is not large enough to support middle-men in this business. You have to do all by yourself.
5) (This is related to #4) You should always
look to add value to your trees. Often, raw material (mostly yamadori) can be purchased significantly cheaper than finished bonsai. Here the 80/20 rule applies: 80% of your profit is created by the final 20% of the work, and this is FINISHING the tree. The first 80% of the work is already done by others (nature, bonsai growers, etc). But you need to be able to do the final 20%. And this is where you can make your profit. I've seen this over and over again: bonsai hobbyists are very good at creating great bonsai material, and then the tree is never finished (A tree looks great at a local show, it has great potential. 5 years later, I see the tree a the same show, and it hasn't changed much - all that potential is never realized because the owner is either not skilled enough to improve it, or he has too many trees and little time to work on them, or he is too much in love with his tree to do anything).
6) You need to
recognize bad investments (bonsai that will never amount to anything) and get rid of it asap. The problem is, bonsaists almost never do this. Trees have emotional values, and we find excuses to keep them ("one day it will be great", "it just needs a little time to mature", etc). Yes, there are trees that will get better, but they are much more that will not. These trees will take up your time and will prevent you from investing and spending time with better ones. You need to be able to recognize and get rid of them, even at a loss (just imagine what would happen to Walmart or any other successful retail business, if they kept all the bad inventory - it would be a disaster to their profits).
7) Finally, you need to
know what the buyers love. Which species are the most sought after. Which is the size, style and shape in vogue. You may appreciate a certain style or look, but that may not be where the money is. You may be sick and tired of the similar-looking, overgroomed black pines, but if that 's what the public loves, that's what you may need to invest in.
So, if you possess ALL of the above skills, you can make money in bonsai. Unfortunately, loving this hobby, being an artist, or loving trees, is not enough to make a dollar. Just like loving great food, and being a great cook, has nothing to do with making money in the restaurant business. Being a businessman is just as important.
The bonsai market is a very fragmented and inefficient market. Like in any similar markets, those who have the information, can make money. One can buy a bonsai at one place, and sell it at a great profit at another place - because the buyers and sellers have very limited information, and those who do have the information, can profit from it. But to do so, one needs to be very market savvy, sometime ruthless and thick-skinned. Unlike in other businesses, where seeking a profit is the normal thing to do, the bonsai community doesn't look kindly at people who are in it for the money. The word of the day is "sharing", "volunteering", "sacrifice", "teach" and other similar altruistic motives. "Profiteering" is looked at as evil.
Sorry for the long post, but these are my observations, after following the bonsai scene here in California for the last 15 years.
P.S.: Since most of us don't have all the skills listed above, I agree with the previous posters who said:
The only reason to invest in bonsai is not the money. It is because you love bonsai.
Is it realistic that we get rich from investing in bonsai? Not at all, we love bonsai too much to look at it as business enterprise. But it can be realistic that once in a while, we make a great investment. Once in a while, we make a good sale and be proud of it. Once in a while, we make a steal that looks like highway robbery. This can certainly make bonsai more fun and we can score some big points with our spouses.
... But then again you never really know what some people will pay until you try. There's a few larger ficus bonsai that a local garden center is trying to hawk for CDN$1,000+ if I remember correctly and they may actually get it. Despite the fact that they are truly abismal specimens and I wouldn't take them for free (in all fairness I must disclose my unbounding dislike of ficus as bonsai in the first place but these things are just plain amatuerish and UGLY). 
