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.
Let say you can have 100 000 trees per square km. If 40 trees gives you 1 ton of carbon per year then spending $25 000 gives you $10/ton.
If you want to convert not-woodland into woodland, then that limits you to farmland or pastures - because there's no such thing as "unused natural potential woodland", any potential woodland that's not used and left alone becomes actual woodland; any potential woodland that's not woodland only became that way when we cut down the trees and cleared the land because we wanted to use it otherwise.
Some countries are increasing the forestation as well https://www.citylab.com/environment/2018/01/northern-forest-...
And then you have a lot of captured carbon dioxide on your hands - next big cost is the storage/conversion.
IPCC summary on cost of forest sequestration :
> Estimates of the private costs of sequestration range from about US$0.10-US$100/tC, which are modest compared with many of the energy alternatives (see Table 3.9 and Figure 4.9). Additionally, it should be noted that most forest projects have positive non-market benefits, thus increasing their social worth
http://www.ipcc.ch/ipccreports/tar/wg3/index.php?idp=171#fig...
The founders of Patagonia and The North Face brands did start conservacion patagonia together. [1]
It is one amazing project. [2]
1. https://en.wikipedia.org/wiki/Conservaci%C3%B3n_Patag%C3%B3n... 2. http://www.conservacionpatagonica.org/aboutus_oh.htm