If I make %45k and Im retired, and I sell a lot of land for $300k in 2025 with a cost basis of $50000, how much do I pay? Federal and state, in taxes?

Too many unknown factors.  You can try the federal or turbotax estimators.

https://www.irs.gov/individuals/tax-withholding-estimator

TaxCaster tax calculator

 

Assuming you are single with $250,000 capital gains, some of the gain will be taxed at 15% and some at 20%.  But the gain may also adjust the taxability of your social security.

 

Most states don't have a preferential capital gains rate and your gain will be taxed as ordinary income.  State taxes range from 3% to 13% depending on the state.