You can also craft higher level quantums at Memory Alpha. With the Season 3 update that goes live today MA will get dozens of new items and a revised crafting system. So that might be a good way to get what you want and probaby cheaper than on the exchange. Quantums are always in high demand there and usually expensive. Quantums are the right way to go if you use well timed torpedo volleys with high yield. They offer a big punch if you can get them directly to the hull. If you just wanna spam your enemy with torps go with photons. You can fire them faster and without high yield the difference in damage is not that great IMHO. Not sure right now if Skillpoints for photons come cheaper than for quantums.