#1- Is this a family or "valued" place, or is it "just land"??
If "just land", may be a good time to plan on ALL of you selling, and 1031 your individual proceeds into a place you like better (on your OWN).
If it's "family land", or you just really like it, then get an appraisal (you can get an initial "general" idea by searching your county on Land.Com, and looking at neighboring land values, so you AND your relative has an idea of what to expect).
Also, as previously mentioned, NOW is the time for you and the other "remaining" owner to have some REAL discussions, and some written agreements, on what happens in the future. You two may get along great, but once spouses/children get involved, if one of you dies, it can go south FAST. Make sure there's something regarding what a future buyout would look like (appraised market values, with the other party having first refusal, not some "lottery" value that one of the kids believes), or divide the place up, if possible to do so, and agree on some visitation/use/hunting rights, if you want.
Also, I don't see the future going well if YOU own 2/3, and the other person owns 1/3.
Discuss with the seller about the benefits of a 1031 exchange, if they don't already know. It COULD give you more time to come up with the money, but if they DO want to consider waiting 6 months-1 year, to do the buyout (to find another piece of land), force them to agree to a price NOW, and contractually hold them to it, so they don't expect ANOTHER 10% or more, if values go up in a year…