Heres something that might help. Thats a census of roman coins in mjor collections - it shows how many of each emperor they found in 200,000 coins. It shows relative rarity for each - its not correct, of course, for several reasons, but the gist is there. The way of telling rarity is anyone below 80th rarest is worth no special premium (with the exception of several emperors like Galba who are worth more due to historical context. Also, misplaced Emperors like Jovian). Above 100 you've got a rare person, above 130 you've got an actually rare person, once you're at 150 you've got unobtainable coins - very expensive.