They will be doing a mix of peering both across free PNIs and very low cost IXP ports, with the reminder going down transit like Colt or cogent. Probably average cost of the order of about $1 per 20TB of egress in Europe and NA markets.
The thing is with edge capacity is that you massively overbuild on the basis that;
It's generally a long ISH lead time to scale capacity (days not minutes or hours).
Transit contracts are usually 12-60 months
Traffic is variable, not all ports cover all destinations
Redundancy
So if you are doing say 100Gbps 95%ile out of say London then you will probably have at least 6+ 100Gb ports, so you do have quite a bit of latent capacity if you just need it for a few hours.
The thing is with edge capacity is that you massively overbuild on the basis that;
It's generally a long ISH lead time to scale capacity (days not minutes or hours).
Transit contracts are usually 12-60 months
Traffic is variable, not all ports cover all destinations
Redundancy
So if you are doing say 100Gbps 95%ile out of say London then you will probably have at least 6+ 100Gb ports, so you do have quite a bit of latent capacity if you just need it for a few hours.