Actually I assumed that people wouldn't be able to pick their specific port numbers - they'd request a port and be assigned an arbitrary one. Since there's around 64,000 available ports per IP address then this should allow quite a bit of capacity. Some form of NAT could be used to translate these to different ports in the virtual machines, which would allow applications to stick to the port numbers they're used to. However, it's a pretty poor application which doesn't have a configurable for the port number.
I could still quite understand it being a premium feature, of course - there's likely to be quite a bit of implementation still required, and even just the administrative overhead would be a pain. Also, having to pay even a little bit is probably one of the most effective ways to stop abuses - perhaps you pay a certain amount per 5 or 10 port numbers allocated, for example.
Of course, being able to pick your own port numbers is also a potential premium feature but then you're getting quite far on the road to complete VPS solutions, and I would imagine that's going to incur a higher cost (if it's even implemented at all).
My suggestion would be to implement just being assigned port numbers initially and see how it works out, but it may be that the direct mapping is easier to implement for some reason.