grammatical number - Apostrophe on plurals of a word abbreviated with a symbol?

I can't find any good resources on this topic. We have an application that will abbreviate words at certain screen sizes and uses nomenclature specific to our client's fields.

One question that popped up is whether or not words abbreviated by a symbol use an apostrophe when plural.

The word in question is "dollars", which is abbreviated with "$". It's usually referred to as "{entity} Purchase Dollars", and it's a title, so title-case.

Should the abbreviation be "Purchase $s" or "Purchase $'s"?

And yes, I agree that "Amount" would be better, but "Dollars" is the nomenclature and makes sense to our clients.

Answer

Here’s a late, but possibly more thorough answer.
The answer to your general question is that the apostrophe is used to create plurals of single characters (cit. Wiki). So “p’s and q’s” is correct, or in the strange case of counting dollar signs and other symbols, I could write that there were four $’s, three &’s, and five @’s.

But in the example you cited, the previous answers are correct that the dollar sign functions as either a singular or plural noun, depending on the context. $1 = one dollar, $2 = two dollars.
Your specific case would be “Purchase $,” which looks a bit strange, but is correct.
Possibly “Purchase $$” would be more visually appealing, without breaking grammar rules.
My company also puts out communications using “$’s,” which is how I found this page.
Now I have to go tilt at some windmills...