ListExtensions.cs

using System.Collections.Generic;

public static class ListExtensions
{
	public static T GetRandomWeighted<T>(this List<T> list, Func<T, float> selector)
	{
		T selected = default(T);
		float totalWeight = 0;
		foreach (var item in list)
		{
			float weight = selector(item);

			if (weight <= 0)
				continue;

			totalWeight += weight;
			float prob = weight / totalWeight;
			if (UnityEngine.Random.value < prob)
			{
				selected = item;
			}
		}
		return selected;
	}
}

/*
// get random item with weight
public class Item
{
	public GameObject someData;
	public float weight;
}

List<Item> items = ...;
Item item = items.GetRandomWeighted(o => o.weight);

...
// get random float with its value as weight

List<float> floats = ...;
float f = floats.GetRandomWeighted(o => o);
*/