1. Yes, there is a fee, but I don't know what it is (may be the differnce between the two versions). The minimum user count on the SQL version is 3 and it is a network version, so that in itself will cost more.
2. Using the same server is dependent on the resources available and transaction volume. Other than that, I cannot see any technical reason why you could not. The other consideration is whether FAS can be within the same database ""instance"" as others (because they are compatible) or whether it needs its own ""instance"". In either case, it would still be the same SQL server.