Zen Armaments Fusion engines using Deuterium Reaction Impulse Propellant™
When hydrogen bombs became outdated, a lot of private enterprises of the Black Syndicate' white-collars suddenly ran out of things to use their manufactured heavy hydrogen for. Then, people started wanting to go really, really fast without much effort - so, they started mixing it with other stuff, finally developing an ideal mixture and mechanism to allow relatively safe fusion reactions to propel ships forward. Sure, the occasional detonation did occur and cause widespread radiation poisoning, but for the most part, this happens less than three times a year.
Expensive Zesuaium drives can be purchased, but most are made of various alloys with varying resistance to heat, torsion, and radiation. One thing they all share is the ability to contain and direct thermonuclear fusion reactions, which use deuterium (Hydrogen-2) to create a really big explosion. Since radiation is a big issue with these, their use in atmosphere is strongly discouraged (but people do it anyway).
The drive has to be replaced fairly often; about once every 5 years of regular use for most common drives or every 20 for Zesuaium. The price of Deuterium varies greatly based on who you know and how many slaves you own, but someone acquainted (not friendly) with a factory owner might fetch a price of 10,000 KS for a month's worth of fuel.
The drive also acts as the main power core for most ships, since fusion reactions do release a lot of energy. Generally, the ship's computer systems, life-support, and lower-requirement shielding systems are powered by this, as well as anything else the designer decides to plug into it. Alternative power sources would be used for something like the SEAS which requires a large amount of energy over a short period of time.