Spend all the money really necessary to achieve your objectives. Everything else is overhead. This disease is often found in engineers who won't release products until they are absolutely perfect. Following this rule to its logical conclusion, finishing the last 20 percent of the last 20 percent could cost you more than you spent on the rest of the project. When it comes to product development, Zeno's paradox rules.
Perfection is unattainable and very costly at that.
Plus, while you getting it right, the market is changing right out from under you. On top of that, your customers put off purchasing your existing products waiting for the next new thing to roll out your doors. Focus on creating a market-beating product within the allotted time. Set a deadline and build a product development plan to match. Know when you have to stop development to make a delivery date.