the royalties, as he says, are "possible", so there are some random on how much finaly u got for the film selling to the "market"

u must watch at ur films stored (ended on selling time) the PROFIT, it shows the real profit from that film (total income - costs)

the profit, as marcus says, depends on the final score (that depends on ratings, actresses, skills, ...)

but TOO has random on it

u can see 2 films sold with the same rating and one got double or triple profit

that too depends on how many films are on sell at same time and the factor "random" (i guess simulate public taste or so)

anyway, to film pro, with a decent score, is one of the best profit business in the game