In a monopoly, prices are usually higher because there's no competition, whereas in a competitive market items that are not priced accordingly may never sell.
For example, if you are the only bread-maker in town you can charge whatever you want - if people want bread they have to pay your prices, period. But in a competitive market where there are 20 other bread-makers, your prices have to remain competitive with the other 20 or no one will buy your bread.
