There might be some more exotic spices that would be worth even more - cinnamon and cardamom, for example.
I agree with guns, but knives are very subtle - they have an appearance and purpose that has not appreciably changed over time, even if the quality has.
If you want to go anachronism, and have the least cost, and the most possible money - how about a bag of cheap solar powered calculators? Sure, you have to teach them arabic numerals, but the calculating power available in a $0.99 solar calculator is incredible, relative to what they had.
The problem in Roman times (I’m on a Roman kick now, reading about the empire for the last year or so) is that you would need to be able to get your way into society to be able to survive. You would need to be able to be in the “in crowd”, or your life would be in danger, as unscrupulous and jealous enemies would have you killed to get your calculators. And it wouldn’t have to be something as blatant as a frontal attack, for those of you Kalishnakov-happy persons out there. Poisoned food and wine, poison gas, scorpions in the bed…eventually, they’d get you. Plus, if you had 1000 bullets, they’d send 10,000 men to get you.
You would need to get a patron at a very high level - almost certainly in the Imperial family. You might do better in the later empire, when one of the Caesars might use you as a “secret weapon” to take on the Emperor (yes, the two words are not the same). You would need a lot of wealth, and some good links in business and posibly the army - maybe you would even have the goodwill of a local garrison by giving them Holiday Presents - there is so much to do!
And I read Lest Darkness Fall as well. The alcohol idea might work, but I wonder about it. It seems to me that from my reading of the last 600 years of the Empire, that what would have saved Rome more than anything else would have been a long series of intelligent, non-criminal, sane Emperors. No technology needed - when Rome was flourishing, it did flourish. And when it sucked, it sucked.