1. One month into the set prices for everything are at their lowest because of heavy early drafting. I buy from bots as much of the set as possible, with the exception of any mythic rare that is way expensive but is useless in modern (Garruk Apex will likely be one of those since anything at 7 mana in Modern needs to say in the rule text "You play this card, you win the game"), or cards like the Scry lands, that, while useful, will never be $5 useful in Modern.
I also buy all the cheap cards at this time to simplify my completion. It is a burden to be aware of 100 cards you are missing. If all of them together cost you $5, even if they will be at $2 by the time the set is about to rotate, it makes for much easier accounting to get them out of the way, and that way you can concentrate on the handful you are missing.
2. As time goes on some of the money mythics or rares go out of favor. I pick those out one by one. Because I also play a lot of constructed, if I like a card and want to play it, I will buy it, even at a higher price, as long as it is under $5. I did this with Ashiok, even if this card will likely go down more as it nears redemption. I love Vraska, and once it went down enough, I bought it and played it tons.
3. Whatever is left is usually 5 to 10 cards (2 in the case of DGM). If I am unlucky, as was the case of Liliana of the Veil, I bite the bullet and buy it for my redemption. Once I am down to one or two cards, like Voice of Resurgence, I will do a draft or two to see if I get lucky. In the case of VOR, I did, and got mine out of a draft.
4. I make sure I am done by October. Some of the money cards will slowly become popular in other formats and will climb out of nowhere. This is what happened with Griselbrand. I got it at $20 as it was cycling out, and Boom! it went crazy a month later. I would not be surprised if Vraska never did go down (yes, its a 5 drop, but anything that says 'destroy target permanent' is good, is very good).
Also, don't fall in love with a card, or hate a card. Make sure you understand why a card is destined to be a staple, and why a card is destined to fail in the eternal formats. Griselbrand could only go up, while Thragtusk could only go down. Snapcaster Mage could only go up, while some of the Theros one-trick ponies will go down hard once rotation nears. I just got Domri Rade for my redemption just in case: a 3 drop planeswalker is a 3 drop planeswalker and Domri has seen tons of play.
