Actually, money-wise these numbers help you a lot, since cult troops are not sold in kits with 10 of each. Plague boys come in 7s, Zerkers in 8s and 1k sons in 9's. Getting 20 1k sons means buying 3 boxes and having lots of marines left on the shelf, unless you are good with converting!
While my quality of skills at conversions is debatable, it's certainly not something I'd shy away from.
Actually I had an idea, and I know this is more in line for the models section then here, but you guys are already reading this. Using Plague Marine or Thousand Sons rules to play a Dark Adeptus unit. I mean with Thousand Sons:
SnP: The marines are mechs and they have difficulty dealing with terrain that isn't flat and level.
The Sorcerer Commands: There's a guy controlling the machines.
I figure the AP3 bolters could be represented by Plasma guns or something similar.
With Plague Marines: The Toughness 5 could be represented by increased armor on the Servitor bodies while Fnp can be used to explain how the machines have redundant control systems.
Thoughts about something like that?