I think in answering this question you should first ask yourself if you're startup is a Technology enabled company or a pure technology company as both have different needs.
An eCommerce firm or marketplace company are technology enabled and are not building a company based on patented software. Companies like Google and Skype are technology innovators as their core value lies in their technology. If you are a technology enabled company then I'd certainly go down the outsourcing route to get your product to market fast whilst you focus on the marketing and building an audience ready for launch.
However pure tech companies like Google could not get very far if they outsourced their entire product since the value of the company is based on technological innovation, but when it comes to building an initial prototype or MVP product to prove the market and demand then that's a different matter.
So for an unproven or pre-funded startup and especially if software development is not your core competency then it makes perfect sense to make use of a top-notch outsourced team with the experience to build your prototype or MVP product quickly, which allows you to focus on marketing. It means you are also not having to spend money on building teams and infrastructure in your business until you are sure you're on to something and have a great product that's going to sell.
What we wouldn't advise however is trying to do this on the cheap as this initial product will actually form the foundations that your startup will be built on so you want to avoid having to rebuild this early stage product from scratch when you try to scale it up. Especially if you are bootstrapping your startup there will be a temptation to cut costs at this point but that will most likely just create issues later if you end up needing to re-engineer or worst still throw away your mark 1 version. So we advise bringing in the best and getting in people who've done what you want to do already and can build you something that will be a solid base to build your business on.
As well as client work we also release our own products and services and have gone through prototype and MVP stages with beta users and customers on standalone and subscription services, so we've done it and built it already and know what's needed. So its well worth getting in touch with us for a chat even if its just for a bit of friendly advise.