Missions IS a low-powered campaign after all, so that people jumping fresh in after a while don't instantly get wiped out. But yeah, availability 16~19 is always going to cost at least 50% extra, while Availability 20 costs 150% extra assuming you have a 5/4 Contact. Loyalty tended not to go past 4 in the past after all.
UNLESS. The Faction Rep included getting an item at a discount before. Maybe if these benefits could also include access to a higher Availability, perhaps +FactionRep? That way, if you suck up to the right corp employee enough, you'll be able to get the nice toys after a while?
I agree there's a bit of a gap where a few toys are now impossible to get unless you pay a LOT extra, and most of those are already expensive enough to require saving up 3+ missions in a row, without paying extra. There's space for amending that though.