I disagree with this on several levels. I'll start by saying that I have no land so it's irrelevant for me. I still disagree and want to give my opinion.
It is mostly irrelevant. Even without an extra cost, I'm very confident less than 1% of the cards working on land will be soulbound. This is coming from someone who plays in diamond/champion. Most of my commons are at level 7 or 8. Most GF commons are a bit lower than the regular foil. The exception is GF Wily Coyotian which is at level 9 with 6 extra copies. My rares are at level 5 or 6. One is still at 4. Some GF are higher level than regular foil and vice versa. Epics are at 2, 3, or 4. Most of them, the regular foil is the higher level. Most of them are at level 2, some are at level 3 and two are still at level 1. I have a few GFs here. So, if I earn at the same pace (this will be important for later), I will surely max most of these cards, in 5 months.
So, assuming a player who's playing at the top leagues should be able to max most of their soulbound cards, is that really a problem? Are they really going to stop playing with some of these cards to use on land when they can buy multiple max copies of cards with similar effects on the market without hurting their gameplay? Unless they have a max regular and max GF copy, this won't come into play. And, if they do have both maxed, they should be rewarded for it!
It doesn't matter if someone has 1000 or 10k accounts farming rewards. They can't put all those copies together. Are they gonna work their lands with silver level monsters? You're saying that someone invested money creating all those accounts, bought all those lands and they are gonna save a fraction of that cost, when cards are so cheap, to use silver level monsters on their land? And if there's anyone with many bot accounts in champion, congratulations, you deserve to use those cards on land.
We're assuming rewards will stay the same for the next months. They won't. Thanks to the brilliant (that's sarcasm) numbers of the SPS requirements, most players will earn a fraction of what they were earning and/or will drop leagues, which also decreases their earnings. Cards will be earned at a much slower pace. And, when it comes to bot farms, the SPS requirements apply to every single account. Are they gonna invest/rent enough SPS for all of them to keep earning at the same pace? If they do, again, they deserve to use their soulbound cards at no extra cost.
The cost is absurd! What worries me the most is that the same logic is already being applied to unlocking the cards to be tradable in the future. The cost should be exactly the burn value, no more, no less. Making the cost equal to the burn value makes sure no new DEC is minted with those cards. That's enough, you don't need to put a huge paywall on unlocking those cards. If you do go with this manifest cost, make it equal to the burn value and, when it's time to make them unlockable, that's already paid.