What’s the meaning of “if that” at the end of a sentence?

I find it often after a number. What does it mean and what is that? It’d be nice if you could help me.

(from Google search)

Some books that are more than 100 years old still won't sell for more than a dollar, if that.

(Harry Potter 2 [US Version]: p.123)

Nearly Headless Nick, the ghost of Gryffindor Tower, was staring morosely out of a window, muttering under his breath, “… don’t fulfill their requirements … half an inch, if that …”

"If that" is a shortened form of "If [it even achieves] that", or some related phrase, like "If [it actually is] that [much]", "If [it actually sells for] that", etc.

It also implies "...but probably not", since "if that" is a strong conditional suggesting skepticism that the aforementioned limit might actually be reached.

So it basically means what the two fellows up there suggested: at the very most (but quite probably not), at the very best (but probably not), at the most extreme (but probably not), etc.

Because of the implied skepticism, the phrase usually indicates that something will bring in, achieve, or result in returns far less than the interlocutor's expectations. An example conversation might be:

"I was hoping to sell this car for US$5,000."

"Oh, no, you'll never sell it for that much. The most you might get for it would be 2,500 -- if that."

Generally, if that means at best / at most.

Some books that are more than 100 years old sell for a dollar, at best.