Buying land and planting trees there will cost something like $2000 per acre and retain something like one ton of CO2 per acre per year (an order of magnitude estimate - depending on details both the cost and CO2 effect can be very different).

Industrial carbon capture at power plants can do that for something like 70$ per ton. That's much cheaper than forestry, but that's still not good enough. ycombinator is obviously looking for technologies that scale better than these existing approaches, something that might achieve large scale carbon removal at maybe $10/ton or less, at which stage the option "just pay a lot of money to reverse the effect of our emissions" might be plausibly considered affordable to our society.