30r-100r for a bottle of zombie virus is what I've usually seen (on smp3) Best of luck in buying/selling your items!
Personally I think the stuff is worthless. Some people might be asking $100 a bottle but I seriously doubt they're getting it. Here's and example of what it's worth: http://empireminecraft.com/threads/auction-dc-of-zombie-virus.38772/ And another: http://empireminecraft.com/threads/auction-dc-of-zombie-virus.38772/#post-737533 I throw all mine away. Sorry if I disappointed you.
I personally think they're worthless, ever since they were nerfed they have no use except wasting chest space.
It's a cool Idea the just need to make some changes to it. It would be cool if they made it strictly cosmetic. If you drink one you become an enraged zombie for 1 minute.
I believe 20r to be a good price. I personally find them quite useful in Momentus battles, but that could just be me.