I think the hobby aspect is a big reason that a lot of people got into it. The problem is that it's starting to get to the point that you really can't just have a video card or a CPU running and expect to make anything significant. Could you make a few bit cents every month? Sure, but there's a few things you (and anyone else that's new to BTC) needs to understand.
1 ) The amount of BTC your mining equipment will generate will go down over time as more people get into mining and the difficulty goes up.
2 ) The cost of power your mining equipment will use will stay the same, or even go up over time.
3 ) The reward is about to be cut in half to 25 BTC. That happens every 210,000 blocks (which takes about 4 years), and is probably going to happen in the next two weeks, so the amount of BTC your equipment will generate will automatically be cut in half.
4 ) There's a very good chance that ASIC's will arrive soon and pretty much blow GPU/CPU mining out of the water. A really good/efficient CPU can generate .1-.2 Megahash for every watt of power it uses. A good/efficient GPU can generate 1.5-2.5 Megahash/watt. FPGA's are up next with 20-25 Megahash/watt. The next evolution are ASIC's and there's estimates that they will be able to generate upwards of 1000 Megawash/watt. If they do hit, difficulty will skyrocket over a month or two and could easily end up 20-50 times higher than it is now.
5 ) The $ / Bitcoin exchange rate may or may not increase in value as the difficulty goes up. Honestly, this is almost impossible to accurately predict. One would hope they would increase in value since it's harder to get them with the reward cut and difficulty increase, but you never know. There's very good arguments that BTC could be a $1000 per coin in a year, and there's very good arguments that they could be worth $0.01 per coin in a year.
6 ) Do not spend any money on mining equipment or buying BTC that you can't afford to live without/lose.
7 ) You can easily approximate what your cost to mine and what you'd receive in bitcoins will be right at the current moment, but never assume that will be what you can do long term.
8 ) The next 2-3 months are going to be a wild ride if ASIC's do come into play and it's anybody's guess where things will end up.
So, for the calculation part, here's a simple example that you can run through.
First, check and find out how much you pay per kilowatt hour for electricity from your power company. Once you have that, determine how much wattage your 'mining' equipment will be using. You can get a cheap Killawatt meter and see exactly what you are drawing from the wall, or you can just guesstimate by looking online to see what your mining equipment will use at full load. Once you have those pieces, it's a simple matter of math.
(Wattage Used * 24 (hours) * 30 (days)) / 1000 = Kilowatt hours used/month
Multiply your Kilowatt hours per month times how much your power company charges per kilowatt hour and there's your power cost per month.
So, for our example, say you are mining with a Radeon 6770 which can be found online for about $90. That card will hash about 200 MH/s and draws around 110 Watts, so...
(110 * 24 * 30) / 1000 = 79.2 Kilowatt Hours / Month
Now let's say your power company charges 10 cents a kilowatt hour (which is actually pretty low).
79.2 * 0.10 = $7.92 Power Cost / Month
At the difficulty/conversion rate right now, those 200 MH/s will mine approximately 1.82 BTC/Month worth about $19.97
$19.97 - $7.92 = $12.05 Profit / Month
Assuming nothing changes (which as we know, is not going to happen), your payback on that card would be 7.5 months, and it will likely be much longer as you will be generating less BTC over time.
When the reward cut hits in the next two weeks, that card will still use $7.92 worth of power, but only generate .91 BTC/$9.98 worth or a whopping $2.06 in profit, making your payback on the card now 43 months+
To make matters worse, if ASIC's hit and difficulty jumps by 20-50x, your BTC generation drops by 20-50x as well. We would hope that price of BTC/$ goes up and the BTC you do generate are worth more (hopefully enough to cover your costs), but it's relatively easy to see how you could end up spending more on power than you are generating in BTC. And while it might still be a fun hobby to mess around with, even if you are losing money, at that point you would be much better served to instead buy BTC directly than attempting to mine. If it costs you $5 to mine $1 of Bitcoins, you have $1 worth of Bitcoins. Had you taken those $5 and bought $5 worth of Bitcoins, you have $4 more in Bitcoins that if you had mined.
Sorry if this comes off as condescending or being crotchety, as it's not meant to be that. I'm just seeing a lot of people not doing any research and coming in thinking, "I can print money with my computer and become rich, sweet!" Unfortunately, mining Bitcoins doesn't work that way, at least not anymore
