So if you have 25% curation being paid out then a $2 vote pays $1.50 back with 50% SBD (0.75) and 50% SP (.75/Current Steem Value or .44SP). The math isn't exact, but close enough. So you get say 1.19 in return on that $2 vote. So that would take would mean you need to pay around 0.71 to break even.

Now this is all complete nonsense as very few posts actually pay out the full 25% in curation rewards. If even a single vote is cast in the first 30 minutes makes the 25% not happen. 

If you gain a couple votes during the first 30 minutes then you are looking at even lower curation rewards. I've seen some as low as 10%. 

Plus right now vote bots are providing more value as so many people are panic selling. This means there is less out there to be used for buying votes. I've seen some the last couple days close with 20-35% positive ROI. I still haven't gotten one like that, but would be nice.