Monopoly: Strategy in trading --how do I compute the expected value of a trade?
I don't think there is a quick method, but there are a few simple rules to consider when trading your cards.

1) How much is the card worth to you. Therefore, what can you do with the money you gain as opposed to what could you do with the card you are selling

2) How much is the card worth to your opponent. If you are selling a card that allows an opponent to complete a row, you have to understand how likely they are to build houses/hotels and the likelihood of it affecting your game plan further into the game.

3) If selling to an opponent who doesn't really need the card, consider the likelihood of them selling the card on, and the impact this will have, as per point 2.

For buying a card, the rules are pretty much the same as above, but inverse. You always need to weigh up the benefits you gain as opposed to what your opponent may gain, and think several moves ahead.

Also, take a look at this thread about the best ROI for properties. Which property group colour gives the best ROI , undeveloped or fully developed, in Monopoly?. This will help you to calculate the real values of the properties in the game.