Out of curiosity, is there a rollover option for the lottery? This way if no one wins, the prize gets added to the next drawing. That could add to the excitement.
Since there are 53,130 ways to choose 5 numbers from 25 (I didn't check but it looks plausible) then for a stake of 10 you'd expect the prize to be 531,300 rather than 500. It would take a lot of rollovers before the prize was anywhere near a fair value proposition.
As there aren't a vast number of users and the cash is virtual anyway, how about picking three numbers from 10 (yielding 720 combinations, if I remember my rather rusty mathematics correctly), so a stake of 2 and a prize of 500 isn't so far wide of the mark. Adding a rollover option to that would mean it wouldn't be very long before it would be worth considering buying all possible combinations to create a certainty of getting a share of the jackpot