Why not make it so like you can get a robot at level 5 or something
for something like this:
rank 1: 1000 gold- 1 gold per minute
rank 2: 2500 gold- 3 gold a minute
rank 3: 5000 gold- 5 gold a minute
rank 4: 7500 gold- 10 gold a minute
rank 5: 10000 gold- 15 gold a minute
rank 6: 10 platinum- 30 gold a minute
and you could have it so you can only get something like 1000 while offline maybe?