I submit my photography to a number of stock agencies. Below you will find the three I tend to find myself recommending the most. Each has a different pricing structure, so you are bound to find one that fits into your budget. Beneath each, you will find links to my galleries at each site.
Costs are based on a number of factors, the first of which is size. The larger the image you purchase the more costly. So consider the use of the image. You likely don't need to purchase the largest, most costly image if you are going to use it as a small picture on a website, for example. Some sites also factor in the age and popularity of an image.
Shutterstock provides two ways to purchase photos: On Demand and Subscription. On Demand, as the name implies allows you to purchase pictures as you need them. You have a choice of purchasing 2, 5, or 25 images at a cost of $29, $49 or $229 respectively. Alternatively, you can purchase monthly plans that allow you to download up to 25 images per day. Shutterstock also sells stock video, if that is something that interests you.
iStock photo also has two methods of purchasing images: Credits and Pay As You Go. The smallest package, 30 credits, costs $49.99. Pay As You Go tends to be a bit more pricey, but if you are only after one or two images, you can save a few dollars from the smallest package. iStock also sells stock video and audio.
Dreamstime offers both credit packages and subscriptions. Credits are less expensive here than iStock, and you can purchase even smaller credit packages, with the smallest an 11 credit package for $14.99. Subscription packages can be as short as one week or as long as a full year. Dreamstime deals strictly in imagery, so no video or audio here.