You can get a good idea of the worth of your parents' land by finding out what has sold nearby that is similar, preferably within the last 6 months or less. Also look at the asking prices of similar properties and the days they have been on the market. A realtor or realtors can do a comparable market analysis, and/or you can hire an appraiser to give you an opinion of value as well. More opinions may be better and help you narrow it down. It also depends if you are looking to sell, refinance, or just to have a sense of things---the opinions may vary somewhat.
Raw land is very, very hard to estimate; because, until it has value due to the oil or minerals on it, it's vale would be determined by the use the owner would put it to: (never end a sentence with a proposition?)
If it was farm or ranch land; that would def affect its value. But you wouldn't value it by buliding a high-rise on it.
Conversely; if it was located in the city, it would make a good ranch or farm.
So when a Buyer want to buy raw land; whether it is 1/4 acre for one building, or 10 acres for a bunch of buildings; they would need to submit a "plan" for what they intended to do with the land. Otherwise, it would difficult or impossible to get a loan to buy the land.
So, if you value the land at $50,000 for a nice big hacienda, someone else may value the land at $500,000 for a Hotel or shopping complex.
hope this helped