It's the same engine - the street version is detuned to 350ps.
The RS-C WAS worth that, Rick at Racecarparts is selling one for anyone interested in owning one. Comes with a spare engine I believe.
As for cost. Put it this way, you can't buy one in Japan in any condition for less than 3.5m yen - which is £22k at today's exchange rate. That's before you deregister it, ship it, pay tax on it, IVA it, register it, tax it and insure it.
Not cheap.
So not a ridiculous price. Worth it imo. I'd get one.