You can make your money back in 3 months. Because after 3 months you would have mined $129, and the resale value of the board is at least $500 on eBay. So, in essence, anything mined after 3 months is pure profits.

However I don't recommend anyone to buy FPGAs right now, as ASICs are coming in December/January, and they will change the game.

Transactions fees only increase my revenues by ~1%.

What do you think the resale value of that board will be after everyone switches to the next great tech? The long-term trend for BTC mining is always going to be that it is not profitable for people who pay average or above-average for electricity. If you invest capital at the right point in a tech-surge then I'm sure you can see some profits but this business has almost no barrier to entry so you should not expect it to be something you can rely on for profits.

From what I know about BTC, it's engineered so that the currency is constantly operating and "peak mining" - if mining begins to fall off, within a certain number of cycles mining will become easier, thus making mining more profitable again.

The point of that is to keep the rate of block solutions relatively constant, regardless of how many people are mining. After the last big price crash, it was instantly extremely unprofitable to do any mining, and the number of miners crashed overnight. eBay was flooded with 3d video cards. So the difficulty came down again until blocks were being solved regularly, but no one was making any money unless they weren't seeing their electric bill (or had really low local rates).