I've decided to start biking five miles to and from work every day. Is it really necessary to invest in an expensive street bike or would I be okay with a mountain bike around $150 from a retail store?
It'll be adequate assuming you know the bike basics and can adjust brakes, shift mechanism and ride position as necessary. You don't get that sort of thing from a big box store.
Also be aware that the bike will be heavy, and won't stand up to hard use.
I would not get a mountain bike to commute with, the knobby tires and cheap shocks will eat up most of your pedaling energy. If you must I would get this bike. It will be a lot better to commute with and if you don't like it return it in 90 days and get you money back.
It'll be adequate assuming you know the bike basics and can adjust brakes, shift mechanism and ride position as necessary. You don't get that sort of thing from a big box store.
Also be aware that the bike will be heavy, and won't stand up to hard use.