There is no cost to port a US phone number to Twilio. We do have a one-time porting fee for international countries, and it depends on the country and the type of number. For the one-time porting fee for non-US countries, you can review this Salesforce form. 

Please note that in order to port in any country, you are required to have an upgraded project with Twilio. Once the porting process is complete, your ported number will function like any equivalent number purchased from Twilio and will be charged to your project on a monthly basis. 

For any additional assistance, please reach out to porting@twilio.com.