Grab and adapt
Almost all startup ideas derive from existing concepts. A little enhancement, different pricing, service, UX, or particular approach can be the Unique Selling Points (USP) that will lead to a unique product. The same service, but promoted with a different marketing approach, can result in a totally different product. It is a trend that you often see. For example, I have been working on a project for an enabler of Mobile Virtual Network Operators (MVNO). Their customers were all mobile service providers who did not own the infrastructure themselves. The biggest difference between all the virtual providers was just marketing strategy. Anyone can start their own network from scratch with little investment. All you need to do is grab and adapt. You have to be careful, though. You need to be aware of patents and copyright issues, but there are many (open source) projects that you can use as the foundation for your app, or just for a first MVP.
For very little money, you can also buy a concept, or even a complete app, that comes close to your startup idea. In that case, all you need to do is to enhance it and see if you can make a difference. To start something new, most apps just need a little twist. For example, have a look at http://codecanyon.net. You will find some great starting points here. There are apps for Android, iOS, web-based apps, and many more. You can find Flappy Bird clone apps, restaurant apps, and everything in between.
Let's say your startup is developing a travel app. You can search for these kinds of apps and buy one of them. You can quickly prove your hypothesis just by making some modifications to the app:
That is a real time-saver and it is worth the little investment. What you learn will help you define what the app should do and what it should look like at the early stage. Perhaps later you still want to develop your app more or less from scratch. However, if you think that a readymade base app will help you collect useful insights quickly, then it will not be a hard decision to follow this approach.