Assuming election meddling is the thing you want to avoid, even a 50% tax wouldn't take this out of the price range of even the thriftiest of governments.
>By some accounts the Russian fb spend was under $100k
If true, that would indicate that these ads are dramatically under priced. If it was simply a matter of spending money on facebook, things will balance next election, as the price of those ads will go up dramatically
Of course, the fact that Clinton spent so much more on facebook and still lost the electoral college would indicate that if the facebook ad campaign was decisive, that it's more about how you use it than about what you pay Facebook; I would bet rather a lot that Facebook is spending a lot of effort right now to reverse engineer what the Russians did so they can sell it to their more wealthy clients.
You're comparing apples to apples when this is an oranges and apples situation.
Regular campaign advertising (like the stuff done by Clinton in the last election) is at least partly constrained by things like the truth and the semblance of accountability. I.e. if I run an ad saying "vote for me I went to harvard" there would likely be a major scandal because I did not in fact go to harvard. The words came from me, and they were lies.
A 3rd party, dumping thousands of fake news stories on to facebook, disguised as real news and using a little ad spends to pump them up - is not constrained by the truth, and because they are not easily linked to the candidate there is no accountability.
If you're allowed to hide your identity, lie, and disguise your lies as valid information, yes of course you can be more with your ad dollars than if you have to be accountable for what you are saying.
There may be some accountability when it comes to what you say about yourself, but there doesn't seem to be much accountability when it comes to untrue things said about your opponent anymore, though I think it's still traditional to use a third party proxy to say those things:
Assuming election meddling is the thing you want to avoid, even a 50% tax wouldn't take this out of the price range of even the thriftiest of governments.