There are a lot of factors to take into consideration. I have to agree mostly with what @djgigabyte said.
If you are collecting modern, you have to get your promos out of the way because for anyone who tries collecting master sets from a decade back, you quickly learn that scarcity comes into play when you are trying to collect some random common promo that was only released in Europe McDonalds/ Toys R US, an Australian blister pack, or a Canadian EB Games/Costco… these cards can be a pain to get a hold of and are by far some of the most difficult to obtain.
If you are collecting modern, I would actually disagree with what many have said and say that more often than not, do not get the most expensive cards first. Many cards are more playable in the tcg and will drop in price after a year or two (If you want to wait). Other cards like V Charizard or Holo Rare Charizard, typically have initial high price points but then drop due to supply in the market. Other more top tier rarity set cards start high and never really come back down…so take your best guess. Some of the indicators that a card might not come back down is: artwork, pokemon character classics (charizard, blastoise, venusaur, pikachu, etc.) , cute girl trainer, etc.
If you are collecting vintage, I would agree that you should get your most expensive card first. But that by no means is an absolute. Just keeping up to date on how the market is behaving will be your best indicator on when to buy. I’m glad I held off on get a few of my wotc 1st edition cards as the price has finally dipped back down after about two years