I think your BEST bet would be to surf trade sites to pick up used minis. Best way to make a budget stretch.
Do you have any Greenstuff skills? If you can sculpt moderately well, you can pick up a box of Empire Flagellants from Fantasy and make them into Wracks with a simple mask added on. Old Dreamblade figs (or Confrontation figs) purchased from OOP collectible mini sites are super cheap and make phenomenal Grotesques.
If you want to stick with new minis, some Venoms would not go amiss. It'd let you drop some Venomspam onto the table, which would give you some anti-vehicle capacity (which you seem to be sorely lacking).
I also think that you could convert one of your Cronos to a Talos, convert a Haemonculus (or buy one) and run a Dark Artisan formation, which is always a tasty addition to a list.
If you want cheap scourges, get ahold of (or convert) the bits you need for heat lances or haywire blasters. Reaper sells packs of wings (I liked the fairy wings, but I think they also have feathered ones) that are half the cost of a pack of scourges.
Finally, you can always find people to trade other armies/other gaming supplies for excess minis they may have.
(I'm a huge fan of gaming on a budget. If you have any questions or ideas on how to go the cheapest route possible, feel free to message me, I'd love to talk shop!)