Bold Memberships works by connecting to a third-party payment gateway provider (Stripe and/or PayPal) and passing your customers' subscription information onto them. The gateway provider then charges your customers on a recurring basis, using the information provided at the time they subscribe.
The only way you're able to charge a different amount than your recurring plan cost is to manually change the customer's plan. The customer would then have to accept the new billing on their end in order to continue being charged.
For example: You want to charge $5 for the first month, then $10 per month afterwards; you set the plan cost to 5 dollars per month when creating your plans. Wait until the customer has been charged for their first month; then manually change the customer's billing within Memberships under the Memberships tab.