How much should 20 TH/S be worth and what is the ROI for that amount in a the 12 month time they give you to rent it out for?
1 MH/s costs $22 for 1 year only. For yesterday it gave you 0.0017708 ETH ($0.08). The day before yesterday this was 0.00177204 ETH ($0.08).
At Genesis Mining:
1 MH/s costs $29.99 for 2 years. For yesterday the profit is not yet known, but the day before yesterday this was 0.00156047 ETH ($0.07).
At GM higher hashrates will give you a discount.
In both contracts there is no additional electricity/maintenance costs. Difficulty will rise, profits dropping down almost every single day, so reaching ROI with Hashnest is almost impossible. At GM you have a little chance but still very little profit.
Wasn't Ethereum supposed to switch to POS before the 1st of November this year?
Yes it is. But I don't know how this will effect these contracts, because it's quite a change.
They ll probably switch to the next most profitable GPU mineable option. However, there s no proof ETH will switch to PoS this year. There were lots of rumours of it switching last year and nothing happened. I guess it ll take place once Vitalik s got a big enough stash.