To summarize, buying an inexpensive bunk bed is certainly an option, particularly if you make sure the bed is well constructed using quality materials. Likewise, like any bunk bed, it is important to continuously inspect to ensure quality maintenance. Just be sure not to shop only on price, since poor quality beds unfortunately have serious safety risks.
Online beds stores allow you to find products for a much lower price than you would be able to find in a brick and mortar store. It is also helpful to find user reviews from people who have already bought the bed you are interested in. Make sure to find out if the bed officially meets all safety standards.