"Ball park in USD what would the price tag be for a new engine from Rolls Royce? Can an airline walk in 'off the street' and buy one or do they refer you back to the aircraft manufacturer?"
Errrgh. Where would you be walking into off the street if it wasn't the manufacturer? If it was an engine type that was at retirement age, then there may be a few around, or if an airline had just gone under, their spares may be available, but the engine manufacturer is the only real source of engines. Yes, you buy spare engines from RR, not Airbus.
There is a "list" price for engines, but in effect no airline would pay it. Engines are heavily discounted (rumour has it that occasionally they are discounted to zero - I personally don't believe it), especially if there is a PBH contract attached, or if there are competing engine types. Spare engines bought separately tend to be less discounted (the airline tends to be more over a barrel...).