About the value of Petro

Petros most important contribution to the crypto assets market and the new digital economy will be the support offered by a sovereign state.
The Petro is a crypto asset backed by oil assets, but what does it mean?
It is expected that the base price of the Petro (its support price) will be 1 Venezuelan barrel of oil (the price is regularly published in the official website of The Ministry of the popular power for Oil).
The government will not pay in oil obviously, but in its value in Bolivares (Venezuelan national currency). The process is still unclear, but there have been rumours that the government could also buy Petros with other cryptos to support confidence.
The Petro will be traded in exchanges around the world so its price will fluctuate on top of the oil barrel. It's unlikely that it'll be lower than the price the Government would pay for it. It will likely go beyond (maybe a lot beyond) the price of a barrel of oil.

Official Venezuelan oil price, updated every day (in Spanish): http://www.mpetromin.gob.ve/portalmenpet/secciones.php?option=view&idS=45
As you can see, from 2018 the Venezuelan oil price is defined in Yuan, not in US dollars. This is due to latest US sanctions that forced Venezuela to trade in other currencies and one of the reasons to create the Petro.

Example
As of today (February 23rd, 2018), the Venezuelan oil barrel is 369,25 Yuan. The official exchange (also published in the Ministry page) is 1 USD = 6,33 Yuan (CNY).

So, the base price of a Petro would be:
1 $PTR >= 369,25 CNY
1 $PTR >= 58,33 USD

The (purely theoretical) market capitalisation of the Petro would be: 100,000,000 * 58,33 USD = 5,833,000,000 USD

Note: this information is based on the White Paper and on generic statements from Venezuelan officials. Beware that statements are not always technically precise, not only because of the technical complexity, but because the Petro is still under a test phase and will not be actually ready before May 2018. Some elements could still change.