Records have two basic factors that determine their value. The title and the condition. Every record has a different value, sometimes VERY different - based on these two factors.
TITLE - What record is it? Who is the artist? What is the album title and which pressing is it? Many times earlier pressings will be more valuable. Famous artists' records sold in the millions and therefore are not rare. The super expensive Beatles/Prince/etc records you see on dumb TV shows are extremely rare titles by these artists, not the one your brother bought in High School most likely! (Although maybe! haha)
CONDITION - For LPs (The big ones) there are two main factors, condition of the cover and the record itself. For 45s (the smaller ones) it's more about the condition of the playing surface itself only, unless there is bad damage to the labels. The cleaner records are, the more value they will have.
RULES OF THUMB - the rarer the titles are, the more they will be worth - provided there are buyers looking for it. Like all markets, record values are based on supply and demand. Many collectors looking for a title, very few of them to go around = high prices. Million selling artists' records (think Michael Jackson, Marvin Gaye, etc) are often the LOWER value items (with some big exceptions) and lesser known artists whose records were made in smaller quantities are often more valuable. Many times the record we will prize the most from a collection will be something the previous owner had because they were related to or friends with the artist - and privy to a product that few people at the time knew about or wanted. These titles are now very rare decades later. EVERYONE bought "Thriller" - it is a very common record. Even clean copies typically sell around $8.
We will pay a percentage of the records value, if the record is worth less than $5, either because it is a common title or because the condition has lessened its value - we typically will pass on it.
BY THE WAY!!! Don't attempt to clean records before we see them. No offense but you probably don't know how to do it right and you very likely will make them worse! I have some horror stories, trust.