Pretty sure Sardinia-Piedmont was the only Italian country with notable industrialisation in this period, Sicily was mostly still feudal and agricultural. Highly doubt that an island which can't afford a military strong enough to defeat a hastily-scraped-together guerilla force were 'far far richer' than an industrialised, moderately progressive state.
As for his motives, his brutality in warfare hardly shows his motives to be purely for gold. If he didn't care about Italy, would he really have thrown himself and his men against the larger, better equipped and entrenched French garrison in Rome? Even when the situation was bleak and became increasingly worse throughout the battle? There's not much gold in death.
Garibaldi had been known to sack towns and cities, and this is fairly common to an extent in early raiding and guerilla forces because they tend to have little to no lines of supply, and he was certainly a brutal commander, but that doesn't mean he fought in Italy purely out of self-interest.
The Two Sicilies were NOT an island, they were the lower half of mainland Italy plus Sicily. They were not heavily industrialized (they were known as the granary of Europe) but had some highs, namely the shipyard of Castellamare di Stabia, Pietrarsa (largest machine parts factory in Europe), the first steamboat in Europe that used screw propulsion. Not to mention the state subsidized and funded heavily the industrial complex, something that doesn't really scream backwards craphole. Cotton, sulfur, iron, not to mention pasta and edible goods. The Two Sicilies alone could satisfy the entirety of the world's sulphur needs, if needed.
The real problem there was that the british WANTED the Two Sicilies to get utterly destroyed, and this is a very important thing, due to an economical .. let's call it "clash" that happened in the 40s: the sulphur mines were the focus of the problem. At the beginning, more than half of the sulphur in Sicily went overseas, to Britain, due to a deal made by the sicilians with the english, that gave them practical monopoly over the sulphur mines. With time the sicilians realized that was detrimental to their economy and started training workers on their own, on top of giving some of the concessions to a french company, Tax et Aycard. Lord Palmerston, a british lord, told the king of the Two Sicilies that he DEMANDED monopoly over the sulphur mines. This created a diplomatic incident at the next court party, where the king didn't greet the british delegation, preferring the Russian Empire delegates instead. On top of the economical loss, the british also got insulted.
South Italy, the Two Sicilies, HAD to be annihilated for three reasons:
1) the importance south italy would get due to the incoming opening of the Suez canal;
2) the treatment the king of Naples reserved to the freemason lodges existing in its reign;
3) the utter coldness of the relations between the two realms.
It's a long story, but to make it short it was Cavour who pointed out the name of Lord Palmerston to fund the anti-borbonic rebellions.
When the mille landed in Marsala (which, by that time, was basically a british colony), the captain log of the Stromboli (head ship of the sicilian fleet) says that all the british soldiers on the ships -oddly at bay, not in the docks- were dressed in red, like the pirates he was told to hunt by the king. When the Stromboli saw the pirates (the mille) they already landed, and the british shops and houses on the docks raised the british flag under the orders of the ambassador (which lived in Marsala), "to avoid creating an international incident", he said. The brits were so many in the Marsala defense that a guy got lightly injured at a foot and a dog died. The Two Sicilies couldn't defend themselves.
Maybe the fact that a RICH state with a growing industrial economy (but an ancién regime head, not really friendly to jacobines) couldn't face a thousand "hastily-scrapped-together" militia should tell you that maybe those thousand weren't so hastily scrapped together but were indeed a professional mercenary force; maybe it should tell you that the port where they started the invasion had been set up in a certain way by the brits and french so that the sicilians couldn't shoot to stop the invasion; maybe there were powers higher than a lowly mercenary taking charge in that moment of the fate of one of the few last old countries.
The spanish and the british (with the protectorate) made of the Two Sicilies a sad and miserable state, it was only after Charles Bourbon that the country started heading upwards, and it still was before Garibaldi came: in the year 1860 the Two Sicilies had a patrimony of 455 MILLION lire in gold and coin, whereas Sardinia Piedmont had a meager 27 million lire of patrimony.
As I stated before, Sardinia Piedmont was on the verge of economical failure due to the lack of gold in their banks and the great amount of worthless paper they printed to pretend they were bigger than what they actually were. I'll tell you more: the entirety of pre-unitary states of Italy without the Two Sicilies doesn't income to HALF of what the Two Sicilies had in gold.
Gold that obviously disappeared when Garibaldi nominated himself as dictator of Sicily and sedated the revolting (and starving) farmers' revolts with his army.
This trend is even more obvious if you look at the taxation in both realms: the Two Sicilies kept it constant for around fifty years, whereas Sardinia-Piedmont added around 4 or 5 new taxes every year in hope of getting out of the devastating inflation and economic collapse they were about to face.
I could go on for hours on how your view of the Two Sicilies and Sardinia-Piedmont and the entirety of the processes that governed what happened in those years is imbalanced and short sighted, but I think this is enough evidence to make your statement on how the Two Sicilies were poor and "an island which can't afford a military strong enough to defeat the mille" a poorly worded, uneducated, wrong opinion.
