Coffee and Espresso Single Serve

Keurig is the current leader in convenience, variety and cost. There are many off-brand Keurig imposters. The market is flooded with single-serve coffee products compatible with the Keurig universe.
There are a great variety of coffee pods(K-cups). This allows you to buy on various metrics: price, brand and quantity. Pod prices can start at 26 cents each with most around 50 cents and are available from various suppliers and manufactuers in package sized from 24 to 100 easily available.
The machines themselves start at around $40, with the higher end machines priced at around $200

Nespresso universe of espresso products

Nespresso has created a niche in the market as they have specialized with espresso type coffee. Coffee that is more concentrated and some believe better. Though a matter of taste, it is rather simple to turn a cup of espresso into a decent tasting cup of coffee (add water) when compared to turning a cup of coffee into espresso.
The Nespresso universe is locked around Nespresso with very few off-brand products being offered. This results is better quality control but at a cost.
You can find deals for machine starting at about $120 going up to over $400. The pods are more expensive as well. Off-brand pods can be found for about 32 cents with branded pods starting at about 40 cents. Be careful when purchasing your pods as there can be compatibility issuses even amongst the Nespresso branded products.
There are two lines of Nespresso makers. The original line is designed around make espresso and the newer Vertuo which allows for different sized coffee. The Vertuo line is still under patent so everything around it is going to be priced higher.