Is there money to be made attacking vessels with a T1 frigate? You betcha.
First, consider the cost of my typical Incursus. I buy and sell most stuff with buy and sell orders in order to get better prices than are usually going at market rates. Also, these figures don't factor in insurance, which would reduce my costs even more:
Incursus 90,000 ISK
3x Light Ion II Blasters 316,500
1mn Microwarpdrive 27,500
Faint Epsilon Warp Prohibitor 15,000
"Langour" Drive Disruptor 5,000
Micro Auxillary Power Core 45,000
200mm Reinforced Rolled Tungsten Plate 45,000
Hobgoblin II 80,000
TOTAL 624,000 ISK
Now, the loot I manage to extract from the wreckage of my typical target is nowhere near this valuable. I can generally expect to sell whatever modules I may find for a couple of hundred thousand ISK. But then, I don't lose my frigate every time I attack a vessel on one of the belts, either. And there are enough vessels with T2 fittings or a special cargo that all my lost frigates have been paid for, many times over.
The typical mining barge, for example, drops at least one Strip Miner; even the T1 variant goes for over 1M ISK, and many barges fit T2's. It is not uncommon to kill a ship with some T2 modules, whether guns or shield boosters or some such; some T2 drones can be sold for over 1M ISK each. Finally, I have looted a packaged Covetor from cargo (18M), and had the pilot of a Thorax abandon his ship (6M) before I destroyed it, giving himself a better chance of escaping with his implants (he did).
So T1 frigate piracy: profitable. It remains to be seen whether I can continue to make money pirating in a cruiser. It doesn't look promising right now--I've lost 2 Thoraxes in as little as 3 minutes after fitting out the new vessel (jumped by an interceptor while scanning belts). Also, compared to my experience with frigates, in a Thorax I feel like I draw a lot more attention from the hungry pirates higher on the food chain. But I'm just starting out in cruisers; we'll see how it goes after I gain more experience and grow into the class more.