It all depends on what you are trying to photograph.
Your current lens has enough magnification to do a good job on very large nebulae such as the North American and very large open clusters, such as the Hyades. It will also work if you want pics of constellations.
For smaller clusters and nebulae or for bright comets, yes, a 300mm (about 6-8x) lens or similar will expand you capability. Your budget isn't going to allow for much more than the Canon EF 75-300mm f/4-5.6 III
, but that will certainly get you started. Keep in mind, though, it will never be enough for planets or for fine detail in clusters. That type of work requires the magnification of a telescope.
You can do a good job on very large nebulae and clusters with your current