1. Most workshops will do it. You don't have to specialise for a low-level port.
2. How long's a piece of string? Cost ranges from $300-$1500 depending on the job.
3. Yes. There are vastly different levels of portedness
Basically, are you going nuts, or just looking for a little more power? If just a little more, possibly a mild cam, then a basic port+polish will do you fine. About $3-$500.
If you're going nuts, you can look at bigger valves, more portedness
, stronger valve springs, etc. Costs $$$.
Ring around engine/cylinder head reconditioners in the Yellow Pages and get a quote. Usually you fix a price first, then COD.