I suspect many people will keep the full details of their ROI under wraps, but people do share quite a bit so reading threads should help a lot.
The primary variables you want to be concerned with are:
- Cost to acquire the hardware (including CPU, RAM, mobo, PSU, etc.)
- Value of that hardware down the road if you want to get out of mining
- Cost of your power
- Nominal hash rate (be realistic that you won't have 100% uptime with major overclocking)
Secondary considerations would be:
- Cost of cooling (if needed)
- Cost of infrastructure (network, place for it to run, etc.)
- What kind of pool you'll use (DGM, PPS, proportional, etc.)
- Value of your time (often hugely underestimated - but time is more valuable than money)
It's also good to think about what you're going to do with your bitcoins. If you're going to be holding them because they will increase in value then you might consider just buying bitcoins or buying shares in a mining operation. If you're going to be selling bitcoins to make a profit, then you want to take into consideration transaction costs and the potential that price and difficulty may not be in your favor at some point. For example, when prices dropped into the $2 range last fall, difficulty dropped much slower. That squeezed the profits for a lot of people and prompted them to stop mining.
Finally, don't forget the opportunity cost of mining versus other things. Mining takes more time and effort than many people can imagine when they're new to mining. That time could be spend with family, having fun, going to school, advancing your career, etc.
Good luck and have fun!