At the end of the day these submarine cables are just optical cables, that is a really long strand of glass that you can send optical light (laser) down, and it

At the end of the day these submarine cables are just optical cables, that is a really long strand of glass that you can send optical light (laser) down, and it'll pop out at the other end. So the capacity is really based on the equipment at either end (and repeaters if required), the cables can degrade overtime due to hydrogen impregnation and damage from ships etc... your also limited by distance before you need to repeat, however basically there will be newer and faster hardware on the market every day meaning there is no way to work out the total capacity. I've seen unrepeated short span submarine cables only lit/working at 10Gbps, the equipment at either end can support substantially faster (100Gbps+) with some small hardware changes, however simple business sense means your only going to provide capacity for what your customers are only demanding. Prices are also more relational to the traffic going over them cables (and demand) rather than the distance, for example it can be cheaper to get from London to New York on one of these cables than it is to get around the UK.