Low-tech ships burning more fuel does put a damper on their "less powerful, but cheaper" philosophy, because you can burn a lot of fuel in a short span of time. If you spend more than 1/3rd of your time in hyperspace, Onslaught becomes more expensive in fuel, than in supplies. Then there are salary costs...
...well, some quick calculations show that Onslaught is actually the most expensive capital ship. Assuming spending half the time in hyperspace, it's 3% more expensive than Legion (not counting expanded deck crew), 25% more expensive than Paragon and ~50% more expensive than Conquest and Odyssey. Without taking fuel into account, then Onslaught is just 10-20% more expensive. For Paragon to be more expensive, it has to be deployed at least two times a month, in addition to maintenance.
I actually forgot if it was low-tech that was supposed to be cheap on the upkeep and expensive to deploy, or if it was high-tech.
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