Personally a couple of things I've done for poisions are these:
If you take damage to an attribute that is at 0 the damage is instead applied to Con directly.
Posion saves are for half effect, not negation.
This makes poisons highly lethal and thus marginally justify the stupid prices attatched to them. When I don't use the save for half rule all poison costs are divided by usually 50. Most poisons listed are realtively easy to find with proper training, the justification given in D&D for their cost is that they "are illegal and people must get them through difficult sources." I've never found any real reason that someone needs to spend hundreds of silver for one does of deadly nightshade (it's a mushroom that's invariably lethal if consumed) yet that same character can go out and (if trained) find it quite easily where it grows.
I do like the idea that the secondary damage continues each minute, although if you use something like slowblood it's going to be nasty (the secondary damage is death.)