I don't have any T5 resources, but for what it's worth:
- In Traveller: The New Era, maintenance is expressed only in terms of hours, not credits (core book, page 241). For a Jayhawk, this is 89 man-hours per week; if a mechanic makes Cr1,000 per week, that's 0.2% of the vessel's purchase price per year for labor alone.
- In Mongoose Traveller, maintenance is 0.1% of a ship's purchase price per year (core book, page 137).
These rates seems pretty damn low. I'd compare Traveller spaceships to modern warships - loaded with tech, but not cutting-edge like the space shuttle
is was. There are some figures out on the interwebs; someone with a copy of Jane's Fighting Ships can probably get better numbers.
Per a maintenance company, the annual operating costs for a small frigate is between 25% and 30% the construction cost, mostly in the form of fuel and crew; maintenance proper looks to be about 2% of the ship's price, per year.
Per a random defense web site, the USN's estimated total operating cost for the Littoral Combat Ship is about $40 M per year, or about 10% of purchase price (not relative to the class ships, which always cost more). That's everything-in, so if you're charging separately for fuel and crew then 1% of the purchase price is probably close.
Per a major Canadian newspaper, maintenance and operations for next-gen Canadian warships will be 8% of acquisition price, per year. This includes personnel and probably fuel, so maintenance proper is probably between 0.5% and 1.0% per year.
Also of interest, a paper from George Mason describes the annual cost of maintenance for the Arleigh Burke class of destroyer rising by 50% for parts and 100% for labor for routine maintenance (performed by the crew), and 250% for "intermediate" maintenance man-hours (performed at dock, not a shipyard), from a ship's first year to its 16th. So an older ship like the Millennium Falcon or Serenity can be presumed to take much more maintenance than a shiny new Heart of Gold, especially if the owners have skipped a couple of refits.
For a civilian perspective, a paper gives the operating cost for a Panamax cargo ship as $6,500 to $8,400 per day. If a typical bulk cargo Panamax ship costs $20 M, that's about 14% of purchase price per year. However, the point of the paper is that costs roughly doubled from 2000 to 2010, so it seems that this is a volatile factor. An academic page suggests that fuel is the main component here too, with maintenance again making up about 8% of operating costs. The main difference is that military ships have far larger crews; salaries are a small part of a tanker's costs.
For comparison, a modern car may cost $30,000 to buy, and as little as $300 a year to maintain (oil changes, tires, belts, but not gas), so 1% per year. An old beater may cost $1,000, compared to $20,000 in inflation-adjusted dollars when new, and another $1,000 a year as parts keep falling off, so 5%.
The real answer is "do whatever works for your campaign," if you even want to track these things at all - they never named numbers in Firefly, as being perpetually broke but still flying was simply the expectation; quantifying it was needless. If I was going to track this, I'd probably go with 1% of purchase price per year for a brand-new ship, up to an extreme of 10% for an old clunker - 10% of the original price, not what the players pay.