That's not where the cost is. It's in the opportunity cost for that area. Entire forests have been destroyed mostly not for their wood, but to re-use the land for a different, money-generating, purpose.
People are free to buy up land and dedicate it to growing trees. If individuals aren't willing to pay the costs of doing so, then why would organizations representing the interests of those same individuals behave differently? Yes, sometimes small groups can get focused action on these issues, but it doesn't happen consistently enough to be relied upon as the solution.
When you can get citizens to consistently try to solve the problem on the level they can control, then you aren't going to be able to consistently get governments to do it either. The options left are to use educational methods to convince the citizens otherwise or go with options that do not require action level buy in from the majority.
Before there were fire departments there private fire services. You would pay a fee and if your house caught fire your service would put it out. But once your house is on fire to a point beyond your control, the fire service isn't likly to save the building. They'll just be putting out the fire. That's good for your neighbors-- embers traveling on the wind could ignite their rooves. But since it doesn't actually do much for a person themselves, not enough people carried fire service to prevent fires from spreading unnecessarily. So now everyone anywhere with significant population density acts collectively to fund a fire department to keep fires from spreading.
Sometimes the things a person can individually do don't work and they need to act in concert to make a difference. Fire control, transportation, defense... health care? Mitigating environmental catastrophe?
People want to be protected from fires, to the extent that enough will support their government funding fire services. Even with fire services provided, fire insurance is a popular product for those with a sizable amount of assets that can burn down. This has enough support to be enacted.
I suspect the same will happen with climate change once it changes enough. The problem is that by the time directly impacting people enough to convince them of the need for action, the ability to resolve the problem will be outside our scope to control. Much in the same way a few cities had to burn to set an example for others. Which leads us to the other problem, we only have one 'city' to burn.
If we try to solve climate change the same way that we solved fire services being seen as a common sense government action, it will work but only after the city is burning.
The study specifically mentions that they considered land not used, including for growing crops (they did mention using land used for grazing, but since cattle has apparently a significant greenhouse impact, I guess it's fair game)
£50 million is almost a rounding error for the UK government - havering about whether the new aircraft carriers should be CATOBAR or not cost £100 million!