To see how much an auction or other fundraising modules would cost, go to Settings > Billing to see the pricing for each fundraising module.

Aside from the activation fee, there is a $1 per participant fee. If someone bids multiple times, it is still just $1 for that person.

If you're using the text to give or donate feature, there is a 1% fee for collected donations.

If you're not seeing the pricing options from that page, enable the fundraising module in Settings > Settings.