If you're going for a large place then you won't really be able to fill it all up with trials parts, you could easily get away with just a quarter of it for trials.
Fill the rest up with some big ramps, a foam pit and a resin ramp maybe (is that what their called?) BMXers eat that shit up, so there will be tons of them there who would happily pay a fiver each for a whole day session.
That's the most realistic approach to an indoor trials arena I reckon.