Nest egg

Nest egg - an amount of money that has been saved or kept for a special purpose.

Example - "Regular investments of small amounts of money is an excellent way of building a nest egg"

Image - http://a57.foxnews.com/images.foxnews.com/content/fox-business/features/2014/09/04/tips-for-creating-nest-egg-spending-plan/_jcr_content/par/featured-media/media-0.img.jpg/932/470/1452270042215.jpg?ve=1&tl=1