Sales people, especially senior sales people, are expensive for a small company. It's also a hiring area where a lot of hires don't work out -- the people who look good on paper often have massive-company experience and startups are totally foreign to them -- which makes it even more expensive. Without naming names, there are dozens of small companies in Silicon Valley, including those coming out of prestigious accelerators, that have had to shut down because overinvesting in sales made them run out of money.

I suggest you take a look at what Upshift Partners' founder has to say about this; he trains technical- and product-oriented startup founding teams to scale their sales without bankrupting their companies. I've worked with him as a consultant but he's better in his own words: http://firstround.com/article/Looking-to-Scale-Your-Sales-Seven-Bullets-to-Dodge