The best way to buy a good cello on a budget is to seek advice from a professional, such as your music teacher, and to search student models, second-hand cellos and returned lines from trusted retailers such as Musicians Friend or Paul Perley Cellos until a suitable instrument comes up. While some very cheap cellos are available from new, purchasing one from a random online retailer is not recommended as the quality and setup tend to be very poor.
Several other reputable retailers offer second hand cellos as well as a wide range of price points in new cellos, These include Woodwind and Brasswind, and Jim Laab's Music. When choosing a retailer, see if it offers a try and return service, as getting an expert to play the instrument is the only way to assure quality. If buying from a private seller through an auction site or similar, be sure to play the instrument before committing to the purchase to check the tone and setup.
Purchasing the cheapest instrument available is a false economy, as many things can go wrong with a cello, and they can be expensive to fix. The quality of the wood is as important as the craftsmanship, which is why an instrument that is made of good quality materials generally provides a much better playing experience than the cheapest factory pieces. While student cellos are naturally cheaper and easier to play, serious students often upgrade within a few years, so this must be figured into the costs. Some retailers offer hire services that help to defray these costs.