I think you'd have to count the total number of skills you can get on a scepter, multiply that by itself and then multiply that number by the total number of prefixes you can get on a scepter.
For the second type, multiply that final number by the number of suffixes you can get.
This will only apply at a level when all the mods are available to you 100% of the time.
No point to actually making this calculation, since you are only calculating the chance, which means you statistically could get it once every ... but in reality you could be shopping for a milion years without getting one or you could find one first time around(or anything inbetween).
A pally scepter is extremely sought after and as such, worth quite a bit. Though I have not been watching trades lately, a high end scepter was worth ~10 sojs (or more) back in the middle of the season. Right now, I imagine you could get a scepter pretty darn cheap.
Holy shield on a scepter.
Since holy shield is normally not allowed on the scepter, the chance of it being on a scepter is so much smaller then the 'allowed' (1/number of skills allowed at your level).
The thread Ragnarod links to explains it quite well.
The following is a shortened version of how Holy Shield can appear on a scepter, this is by no means complete, you should really read that thread Ragnarod links to for a proper explanation :
While rolling a scepter, it first has to select a 'not allowed' skill for 5 rolls before it will accept Holy Shield on the sixth for any of the skill slots on the scepter.
I wager even a scepter with +1 holy shield should be able to fetch you exuberant amounts of SoJs. No other skills needed on there at all, since it's just used as a buff and saves some characters having to invest the 5 skillpoints to get level 1 in this skill.