Why is the possessive needed for “pounds” but not for “$500”? Thanks.

Because it's written numerically. If it was written out as "five hundred dollars" then it would be "five hundred dollars' worth", but since it's written as just the number and dollar sign the "dollars'" is excluded (but would still be spoken aloud).

I believe it could be written as "dollars' worth" but isn't due to it being shortened to $.

If you wrote out the words, then it would be: *I have five hundred dollars' worth of eggs.*

Because the possessive apostrophe is shown on the s of dollars which is replaced by a dollar sign ($) here. So there’s no place to write the possessive on $500.

Because $500 is read as $500 dollars, or in this case $500 dollars' \[with apostrophe\] worth. But, since you don't spell out dollars here, you just can't see that the apostrophe is already included. $500 dollars and $500 dollars' are pronounced the same.

i don't think it is needed, only acceptable.

Presumably because the currency is spelt out in the first case, while the currency symbol & numeric amount used in the second. If it were "£10 worth", that'd be fine too. But £10's worth / $500's worth just looks odd. Native speakers would pronounce £10 as "ten pounds" and $500 as "five hundred dollars", so adding an additional "s" would seem redundant/confusing.

It is actually in there when spoken, it's just that when written as "$500 worth" the 's is not written. You could write it as five hundred dollars' worth.

That's just a writing / orthography thing. The actual words you say when you read it out "five hundred dollars' worth" have the same possessive form. Orthography is not language.

It’s a common construct. Think of “worth” as belonging to “pounds”. *The worth of 10 pounds*…

Great question! All these other comments are correct: you don’t add a possessive apostrophe to symbols.

It is possessive still, it's just not explicitly shown for numeral form as there's no s to add the apostrophe to because "dollars" is replaced by a symbol

It's not possessive because the dollar sign is used in place of the word dollars'. If you were to write $500s' that would be like saying five hundreds' dollars' worth

https://en.m.wikipedia.org/wiki/Pound_sign#

I personally wouldn’t use the possessive here. Now there is good a chance I’m wrong. But I reckon a lot of English speakers wouldn’t use the possessive.

A tricky thing about this construction is that it's acceptable both as a possessive and not. A good way to show this is with $1 since it doesn't have a plural, making the forms more distinct. "A dollar's worth of goods" "A dollar worth of goods" Both sound acceptable to my ears at least, making the apostrophe functionally optional. Secondly, I think stylistically mixing numerals with possessive markers is probably advised against. "$500's worth of goods" looks somewhat wrong to me and I don't believe I've ever seen it used.

I don't think there should be a possessive for either. This is just a mistake because pounds sounds the same as pounds'. It means that it's worth 10 pounds, not that the worth is owned by 10 pounds.

im pretty sure this is nonpossessive genitive. same as “one day’s journey”. it’s kinda going out of style but not gone yet.

$500 is read as five hundred dollars. So picture “$500 worth” as “500 dollars worth”