There is no magic formula (known) because there are too many different factors. The standard advice is to get an idea what the better teams at your new level would be able to charge if they had your stadium size by looking teams' arena records. This is tougher when your stadium is gonna be smaller than nearly all successful teams though. Another way is to use bb-usa.net to get pricing info (use the graph to adjust higher from what they recommend).
Looking at your prices it seems you have the right idea though. Also you should build more seats.