The Contagion Points system doesn't feel like they actually considered how it would work in Matched Play (which, sadly, is no surprise, Matched Play
still seems to be an afterthought in rules development with
AOS).
As a "house rule" until it gets FAQed, I'd be ok with doing something like adjusting the actual Reserve Points expenditure to meet the unit sizes on the Contagion Points table (so, e.g., half the cost of a 10 man unit of Plague Bearers for the 5 man unit entry via Contagion Points). Sadly, though, right now there's no "official" answer that I can find. I might ping them on Facebook, but they don't usually give official answers there, but it might make it into the next errata.
Edit:
auticus wrote:
Based on
RAW, if you only have the option for a 10-man unit, and you field less models than those 10, you still pay for the 10-man unit.
So if you bring on 5 plaguebearers you'd pay the cost for 10 of them in matched play as it stands right now unless there is some other wording that has not been revealed that I am not aware of.
Yeah, that's what it looks like,
RAW. So it'd be 100 Reserve Points for either 5 or 10 Plaguebearers, though the Contagion Points cost is different between the two.