It is well known that pepper originated in South/Southeast Asia, and this is where it was most available for a long time. Because of the difficulty and time requirement to go from Rome to India and back, pepper was an expensive commodity.

After Rome took over Egypt in 30 BCE, however, trade became much more common, especially on routes that took ships across the Arabian Sea and to Southern India. This became more and more common, and the price of black pepper fell significantly in Rome in comparison to other peppers (such as long pepper). Note that it wasn't "cheap," it was still only for special occasions, but there is evidence that black pepper was widely used in many recipes by the third century. So, eventually, Romans were able to access pepper, but not without a price.