Hi! The quick answer is that simple invention ideas are great as they are the fastest and least expensive to develop, yet can still be highly profitable.
I run a consumer product firm which has developed hundreds of inventions for home-based inventors or small product firms - Essentially we take it from Idea to Store Shelves.
The best advice I can give is to ONLY do what is absolutely required to sell product... There are lots of great services out there that are beneficial; however, if you're on a limited budget, stick to only what is necessary to make a sale, which is: 1. Industrial Design / Engineering, 2. A manufactured sample and a manufacturer who can produce product, and 3. A provisional patent.
Essentially what you need is a real, physical, and fully functioning unit of your product, the prices to manufacture that product and a manufacturer who is ready to produce units, and intellectual property protection so that your idea is not stolen.
Once you have these 3 items, you can start to present your product to wholesalers, retails, distributors, etc. If someone likes it and the price is reasonable, they can place an order, and your business and dream product starts to grow. From there, there is a whole world of possibilities, but the most important thing for now is to develop your product from 'idea' into 'real'.
Whatever you do, do not get caught up in the idea of 'licencing' your idea. An idea is almost impossible to licence unless it is CURRENTLY being manufactured AND being SOLD through stores. If someone or a company says that they will help licence your product idea for royalties, etc., then they should be willing to do that for FREE, no charge, no fees. Many people attempt to charge fees to licence a product idea; however, if you aren't currently selling in stores, it will almost be impossible to get any form of monetary payment, so they usually are just trying to get your fees for their 'marketing' services, which are almost useless for an undeveloped product, and they know this.
If you would like more information on some of the details of product development, we have a free Invention Guide on our website, check it out here: http://www.makoinvent.com/free-invention-kit/
President, Mako Invent