Not all the cost is ink
The sidebar on writing in a wizard's spellbook says, in part:
For each level of the spell, the process takes 2 hours and costs 50 gp. The cost represents material components you expend as you experiment with the spell to master it, as well as the fine inks you need to record it.
So first off, how much of that cost is ink and how much is the materials and such? It's not really specified, so how much of the spell-writing cost you can offset by stealing ink is mostly up to your DM to figure out. We can make a guess, though:
You can copy a spell from your own spellbook into another book — for example, if you want to make a backup copy of your spellbook. This is just like copying a new spell into your spellbook, but faster and easier, since you understand your own notation and already know how to cast the spell. You need spend only 1 hour and 10 gp for each level of the copied spell.
Since there's no decoding or experimentation, most or all of that 10 gp/level cost must be the ink itself. So a decent guess would be that stealing ink could offset at most 10 gp/level in costs.
The ink may vary
However, it may not be as simple as finding a cask of Wizarding Ink and running off with it. Not only is it entirely possible that Warlocks and Wizards use different inks, different wizards may use different inks (in the same way that they use different coding schemes), and a single wizard may use different inks for different spells. It says "fine inks", not "fine ink" -- it's not a single specific material called "fine ink."
Or it could be worse than that. Scribing spells might require a whole set of different colored inks, like you're trying to work in an illuminated tome like the Book of Kells. You might need ink that's mystically resonant with the spell you're writing, like ink made from the ash of a lightning-struck oak for a lightning bolt spell or ink mixed with demon's blood for a fireball. You might need ink that's been steeping with crystals and platinum for six years. Or maybe there really is just one really nice kind of ink that all the wizards use for all their spells and you can buy it by the cask. Who knows? That's up to the DM to figure out, and there are no rules to back it. All we know for sure is what it costs you to write spells the first time and subsequent times.