The answer is, of course, it depends.
Clearly it was very affordable for the middle classes but possibly less so for the poorer elements of society. Most writers suggest that it was the equivalent of something between fifty cents and a dollar, in fact around what it would cost to post a letter today.
But consider Emile L’Angelier, Madeleine Smith’s lover.
In the mid 1850’s in Glasgow he was earning around sixty pounds a year as a seedsman’s clerk. To keep things simple let’s say that he earns sixty pounds a year or five pounds a month, giving him around one pound three shillings a week, which, remembering my predecimal maths would be 276 pennies.
Now, someone working in a similar job today could arguably be getting $60,000 a year (possibly a bit more) which comes out at around $1115 a week.
If we say that a letter costs 1/276th of a weekly income to post, we come to something a little over four dollars.
Now one crucial difference is that Emile didn’t pay tax or a medicare levy. Assuming a marginal rate of 30%, our imaginary seedsman’s clerk would be paying $334 a week in tax, giving him a disposable income of $780 a week.
Using the 1/276th test, that has a letter costing $2.82 to post, not counting the cost of the paper or the envelope, which, in the 1850’s was not insubstantial given that the only paper commonly available was rag paper (Cheap wood pulp paper didn’t arrive until the 1860’s).
In other words, writing and posting letters was a substantial cost. We see a hint of this in some of Madeleine’s letters which are cross written ie the letter is written first top down and then continues at 90 degrees to the original text, like in this other nineteenth century letter
as a way of saving paper, even though as a wealthy young woman Madeleine could well afford to write several letters a week …