I guess we just have differences in play style, but the only battles I've ever needed more than one capital ship for are the battle stations. Otherwise, cruisers hold up just fine and cost dramatically less.
Probably.
For testing purposes, I first waited a good long while for markets to refresh, then assembled a minimal tanker fleet and traveled around all of Hegemony space stripping every market on every planet and station of fuel (aside from a few that never have much if any for sale). It's nearly impossible to avoid or evade patrols with a Prometheus (mine even has Insulated Engines) and a decent escort along, unless you spend five minutes screwing around and hoping patrols wander a long way off, so I had to smash and grab and flee, or occasionally just give a patrol the middle finger.
All told, I'd say I profited about 4,000 fuel, give or take 500. It's hard to be precise, since I had to expend some along the way and wasn't being quite that meticulous, but that's about accurate.
I then reassembled my fleet, but parked one of my Onslaughts and all of my fuel ships, although I kept my cargo ships because it sucks to leave tons of stuff behind if a system happens to be full of metal. This brought my fuel/LY down to 105, which, especially since I have Navigation 3, I think is very reasonable for endgame.
I decided to pursue a fairly close-by bounty that was around $300k. When I arrived, the fleet outnumbered me significantly, and they had two (count 'em, not one but two) Onslaughts. Mind you, this is just a $300k bounty fleet. The $400k fleets are significantly stronger. Maybe the rest of you guys are geniuses at the game and can take down capital-heavy doomfleets with a handful of cruisers and destroyers, but that ain't me.
When I arrived back in Jangala, I was down 800-1000 net fuel, and they still had 0 fuel for sale, having not produced any more. So basically, short excursions even with a pared-down fleet and Navigation 3 cost me 1,000 fuel; long excursions end up costing 2,000 or more fuel, and remember, this is after parking my second Onslaught. That means that every 2-3 excursions I go on, I would therefore need to spend 20-45 minutes either scouring Hegemony space for every ounce of fuel, or else farming REDACTED for a similar amount of time, to recoup the fuel used on those excursions.
This isn't workable and I've decided that, for me, the best solution is to get the Console Commands mod, give myself fuel, and remove the appropriate number of credits from my total. I'm very comfortable with "judicious cheating" to improve but not ruin gameplay from running multiplayer game servers with friends over the years, so it won't kill the game for me.
When 0.9.0 releases, I'll look into creating a proper fuel mod. I know that fuel is there to stop players getting truly absolutely huge, massive killer fleets, but the balance is way off currently. Putting more than one capital ship in your fleet shouldn't break the economy, especially since enemy fleets also contain 2-3 capital ships and usually far more ships than your fleets besides.