There are various reasons both to hire a guide and/or porter, or not to, for virtually any trekker. While the standard route to Everest Base Camp Trek(EBC) is generally easy to navigate without a guide, people do get evacuated due to altitude sickness relatively often, and it is easy enough to get lost if you venture off of the main trail when going via the passes or to a smaller town.
Porter It is a good idea to hire a high mountain trekking
The cost of porters and guides does vary depending on where you hire them. Many guides operate out of Kathmandu, while some operate out of Lukla, which is a pretty hit and miss thing to rely on. Even in the low season, Lukla guides can be fully booked. The disadvantage of hiring guides in Kathmandu is that you will have to pay for their return flight to Lukla, although for them it is half price each way. A good licensed guide costs around US$25/28 per day, which includes his food, lodging and insurance. For a guide/porter you can expect to pay around US$20 per day, including food, lodging, and insurance, and for a porter alone, around US$15/18 per day. Porters can be more easily hired at Lukla, so you can save the cost of air fare for the porter by hiring them there.