[petrochemist]

There are plenty of Cheap long lenses around. Though they won't be of particularly good quality, and will generally be rather slow.
Going too cheap could convince you that you have no use for longer lenses, so Gordon's advice might be the best option. It's particularly difficult to recommend an option when you don't know what it's for.
In good light I've found 500 or 600mm mirror lenses to be useful for motorsport before getting the much more expensive Sigma 150-500. My 300mm f5.6 M42 lens however hasn't been used for around 20 years (film SLR days).