An example of how to easily do an inexpensive alternative gravity setup (untested).

GravityWell.cs

// GravityObject.cs
using UnityEngine;
using System.Collections;


[RequireComponent (typeof (Rigidbody))]
public class GravityObject : MonoBehaviour
{
	public float gravityModifier = 1.0f;
	
	
	void Start ()
	{
		rigidbody.useGravity = false;
		GravityWell.AddObject (this);
	}
	
	
	void OnDestroy ()
	{
		GravityWell.RemoveObject (this);
	}
}


// GravityWell.cs
using UnityEngine;
using System.Collections;
using System.Collections.Generic;


public class GravityWell : MonoBehaviour
{
	public float gravity = 9.8f, radius = 1.0f;
	
	
	private static List<GravityObject> objects = new List<GravityObjects> ();
	
	
	public static void AddObject (GravityObject newObject)
	{
		objects.Add (newObject);
	}
	
	
	public static void RemoveObject (GravityObject existingObject)
	{
		objects.Remove (existingObject);
	}
	
	
	void OnApplicationQuit ()
	{
		objects = new List<GravityObjects> ();
	}
	
	
	void FixedUpdate ()
	{
		foreach (GravityObject target in objects)
		{
			target.rigidbody.AddExplosionForce (-gravity * target.gravityModifier, transform.position, radius, 0.0f, ForceMode.Acceleration);
		}
	}
}