There is a couple of things here. Lets say you offer someone a tech and say "What will you give me for this". Whatever they say is generally the highest value of beakers + gold they will give you. The only leeway you have is switching the tech you want. They will however not give you more than what they have offered. For example, if they are offering you polytheism for writing but have sailing and iron working also, you can get them to trade you sailing (since beakers value is equal) however not Iron Working since its worth more.
The other possibility is the "What do you want for this". Make sure not to get ripped off here, there is almost always leeway here as they will ask for a crapload for not too much, you may still have to give more than what you get, but not the initial price they are asking for. For example alot of times they may ask for 2 high priced tech's, which you can usually switch to 1 high price and 1 low priced tech instead and still get the trade to work. If they definately want the 2 high priced tech's, make sure to make good use of the "What will make this trade work" feature, as they may be willing to give you some other tech's/gold for those 2 high-priced tech's.
