I'm not sure if I'm just completely missing the listings, but is there a source for the cost of Plasma Flask (Heavy), Melta Canister (Heavy) and Fuel (Heavy)? I've looked all over in the IH and the DH books and cannot find them. I've even looked in the Errata v.3.0. Am I just blind and not seeing them or are they referenced somewhere else?

I think you're right about the prices not being listed. DH corebook only lists the price for a lascannon chargepack (30 Thrones), but otherwise only offers the costs for pistol and basic ammo packs / canisters / flasks in regard to las-, melta- and plasmaweapons. Might be because the heavy versions didn't make it into the rulebook but instead appeared in the IH.

After skimming through the IH I can confirm that there're no prices listed for either heavy flamer fuel packs, multi melta canisters or plasma cannon flasks. I'd suggest simply making them up on your own, comparative to the costs given in the corebook. Also, there's nothing mentioned in the errata either.

I had to deal with this problem a while back in my game when a player decided to buy an autocannon. Since the few times such a weapon would actually be permissible for mission use it is likely to be one of those unsubtle knock the doors down with a Chimaera mounted dozer blade I allowed this. Then the obvious follow up question was asked... How much is my ammo going to cost. No listings for heavy weapon ammo costs that I have been able to find so far in the books (other than the items in the DH core book). I ended up pricing autocannon shells at 16TG each, the same as bolts. I reasoned that while the bolt ammo is obviously far more complex to manufacture the autocannon shells are MASSIVE compared to even 75cal bolts, not to mention very difficult to obtain outside of an Imperial Guard armoury. Not the ideal solution, but it seemed reasonable to both myself and the player in question, and his character has a meager Guardsman's salary to blow on stuff like this. Of course since I rarely let him bring anything even half this size on a mission he has yet to require selling his children into indentiture. Good thing too, since he is now Chemgeld as part of the full cyber-rez package.

Also remember that each clip of ammo weighs 10% of the weapon, so it quickly becomes unwieldy with heavy weapons.

What I've used thus far:

Heavy Stubber 25 thrones for 200 rounds (one belt)
Assault Cannon 50 thrones for 200 rounds (one magazine)
Autocannon 10 thrones per Round (its a small cannon mind you, it is still under priced)