That's still only about $1000 in today's $$. A lot of money to someone who isn't doing so well, for sure, but a similar hospital stay today would probably leave you with a hospital bill closer to $50,000.
This also reminds me that in 1958 he sold his father's house for $15,000, but I was reminded by one of the poems on the KPFK CD that his father still owed $8,000 on the mortgage. So Bukowski probably saw five or six thousand profit from that sale, which must have been nice to bank, but it's nowhere near the $15,000 "cushion" that a lot of people (myself included) assumed he had after the sale.
Nothing is free. Back in the not-so-olden days they would throw you in prison for not paying your debts. In fact, the last addition to Moyamensing prison - where Bukowski was held for 14 days - was a "Debtor's Apartment." People in the U.S. and UK were always being thrown into debtor's prison 100 years ago. Whether they still did that in the 1950's is unlikely (though into the 1960's my mother would say things like, "You're going to run us into the poor house!" to my step father...but that's another story).
But knowing Bukowski he paid the entire bill eventually. That letter is dated only a month after his surgery, and the wording of it makes me believe that he wrote them and said, "Hey, I'm broke, please forgive this debt," to which their answer seems to be, "No." Heh.
He was still living hand to mouth at this time. It would be three more years until his father's death and the sale of his house gave him any breathing room financially.