How to add decimal column to existing table in Laravel 5.3 migration

You're trying to use ->addColumn() which isn't quite the right syntax.

Create a new migration, php artisan make:migrate add_cost_column_to_mileages.

Then inside your migration, you want it to look like this for up:

Schema::table('mileages', function (Blueprint $table) {
    $table->decimal('cost', 5,2); //Substitute 5,2 for your desired precision
});

And this for down:

Schema::table('mileages', function (Blueprint $table) {
    $table->dropColumn('cost');
});

This is from the docs here, although they don't make it explicitly clear.