You pay some points to get an ally with points based on a percentage of your own.
If you picked 25%, then it'd be 87 points- about 25% of 350.
The more points it's built on, the more expensive it gets.
Then you have to decide how often your ally will be able to assist you.
As well as any enhancements.
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather