So whenever referring to inanimate objects, "eles" makes sense just like saying "they" in English? Or is there certain rules or times when it changes?
Yes, "eles" fits the case perfectly.
When there is a group of things, all masculine nouns or mixed genders, use "eles".
When there is a group of things, all feminine nouns, use "elas".
In english, objects don't usually have a gender: the table is an "it", the door is an "it", and so on. In portuguese, all nouns have a gender. As such, it makes sense to refer to an object "it" as "he" or "she" (ele/ela). Therefore, when you are referring to a group of objects, you will always you eles/elas depending on what is this group, as danmoller said before. When the group is completely made of feminine nouns you use "elas", otherwise, use "eles".
Yes, your pronouns have to agree in gender and number with the nouns they modify. The only case where you don't have a gender is when talking about a situation or previously referred-to idea (We use the neuter demonstratives isto, isso, and aquilo). Everything else has a gender, whether it be masculine or feminine (and sometimes, but rarely, common gender). Hope that helps!
The audio pronunciation of "mas" sounds different in the full sentence vs when the word is moused over. in the full sentence, it's pronounced as it reads, but when you mouse over, it sounds like "mais". Which is correct?
Following the logic at the Tips and Notes section, doesn't it make more sense to use "estão" here instead? Because what is expensive now can be cheap later. No?