phonology - Is the "an" rule applied when a sum of money is in between?

I have recently seen this image:

Should "a" have been used instead of "an" in the "...an $100,000 apartment" part?

Answer

The /ə ~ ən/ rule, like the /ðə ~ ði/ rule, depends completely on the individual sound that follows. What word starts with this sound, or what its meaning or part of speech may be, does not matter at all.

The rules are very simple to state in their entirety:

/ə/ or /ðə/ before Consonants; /ən/ or /ði/ before Vowels

"5" is pronounced /fayv/, and that starts with /f/, which is a Consonant. Therefore use 'a'.

If it were "8" (pronounced /et/) instead, it would start with a Vowel, and one would use 'an'.

Note that this has nothing to do with spelling, and only refers to pronunciation.