HOMER can model grid extension in a simple way, and compare its cost with the cost of a stand-alone system.  To model grid extension, click the "Add/Remove" button and choose ‘Compare stand-alone system to grid extension’.  Then a button labeled "Grid extension" will appear below the schematic.  That button brings up a window where you can enter the costs of grid extension.  Then, when you double click on a system in the optimization results list, the Simulation Results window will contain a Grid tab showing the breakeven grid extension distance and a graph comparing the cost of grid extension with the stand-alone system.

For details and to see the equation for the breakeven grid extension distance, please look up "grid extension" in the index of the Help system.