It certainly adds some cognitive overhead to the user.

I think a solution might be buying credits, e.g. you have bronze/silver/gold credits, and each article could be priced at 1 bronze/silver/gold credit (because the price being e.g. 3 credits would keep the overhead). This layer of abstraction removes the user from thinking in terms of money (e.g. this article cost 0.25 euro), a bit.

In terms of monthly subscription, I would assume pricing articles individually makes it easy to deal with the publishers? X people read it at price of $Y, so you get X*$Y minus some fee, can't get any easier. It would get much more tricky if trying to split the monthly cost.