Tony, the private room option can vary quite a bit. Our first choice is albergue's with the option of private rooms. We also stay occasional in Pensions, which are similar to very modest hotels that give you a private room, but some times with shared bathrooms. Occasionally in larger centers we will splurge for a hotel. For private rooms, count on about a minimum of 30 Euro a night, 40 being common and some at 50 which is at our high end, but also typically a Pension or cheap hotel. We do shop around and don't necessarily stay at the first place we stop. Also, ask to see the room before you lay your money down.
One thing we found if you stop in towns and villages not typically listed as end points for a daily walking stage, you will get a better deal on your room. Any town or city listed as a
Brierley stopping point are almost always more expensive than non typical stops. As realtors like to say, its location, location, location.
Of course if you stay in private rooms, you will not have much to add when the discussion inevitability arises when people complain about inconsiderate room mates from shared bunk rooms!