For a series I tend not to buy plushies or keyrings, etc and so money can be spent on better figures, manga, and dvds. Generally I think around £200/ $320 to be generous for conventions and events for several series. But I only really go to two events a year and don't shop online for these types of things.
I like a good bargin and I don't just buy the first thing that has my favourite series name on it. I like buying unique looking figures for a good price, despite not knowing about the series. I tend to love the series afterwards.