In evaluating the tiers for you, I would recommend mainly looking at the number of packs and starters, since those actually give you enough cards to try out a few different deck concepts as you start out, and to a lesser extent the mercenary champions. The variety of cards available to you is core to any TCG, but the rewards can get a bit distracting with all the unique cards, equipment, and alternate art stuff. You can mostly ignore those, since you would only be getting one copy of each, which isn’t enough to rely on drawing or build a deck around.
Every tier up to King ($120) offers a better and better price-per-booster in addition to the ancillary stuff. I think the best options are:
Squire ($20): If you get into the game even a bit, you’ll probably buy a few packs, and $20 seems like a pretty reasonable amount for moderate dabbling in an enjoyable F2P game. Basically pay full price ($2 each) for 10 packs, and get two extra starter decks thrown in with the deal so you have more bases to build from, and can experiment with modifying decks right away rather than having to get through an unknown amount of PvE content to build up a decent card pool. You don’t get much free here, but if it’s what you expect to spend anyway, you might as well get the little freebies.
Knight ($65): For around the price of a new-release console game, get 65 boosters ($1 each), along with 4 starters and a primal pack (all rares and epics), as well as a couple of mercenary champions. The primal pack really bumps up the comparative value of this tier compared to the $35 and $50 tiers.
King ($120): 155 boosters ($0.77 each), 4 starters, and a primal pack, along with Spectral Lotus Garden, which is the only exclusive card that I think will be really impactful since you’re not limited to a single copy. And if you decide not to use them, they can go on the auction house to fund a steady drip of income for whatever other cards you’re looking for. Also two more mercenary champions over what Knight gets.
The tiers above King get into more specialized territory, taking the base of King and adding on a special bonus for an extra $130. I think the only ones really worth considering for a PvE player are Dungeon Crawler (double loot and experience) and Pro Player (even if you just draft and drop, the equivalent of three packs per week will really add up and give you lots of additional cards to experiment with). However, realistically I wouldn’t spend anywhere near that much on a game that I mainly planned to play solo.