So to answer your overall question, the main evidence for trade networks is the movement of toolmaking materials over significant distances. Obsidian especially was a material which was traded over significant distances. We use various scientific analyses including isotope analysis and neutron activation analysis to tie obsidian pieces to specific source areas. The two major sources in the ancient Near East were in Cappadocia (southwestern Turkey, just north of Cyprus) and the Bingol region further east in Turkey. Other sources were certainly present, for example, in Khalidi, Lamya. "Holocene obsidian exchange in the Red Sea region." The evolution of human populations in Arabia. Springer Netherlands, 2010. 279-291, it was found that ancient people transported obsidian across the Red Sea from Somalia to Yemen.

This visual set of illustrations by Andrew Sherratt, a well-respected scholar in the field, shows the growth and persistence of obsidian moving from the sources to other places. I believe the ranges of the obsidian trade would be too far for nomadic hunter-gatherers to have consistently moved it. It's too consistent over time in one direction to be reflective of the pattern of hunter-gatherers moving cyclically between known food sources.

The distances involved were quite large. One study on Paleolithic artifacts from the Caucasus region demonstrated a distance of 350 km from source to final destination. Based on Andrew Sherratt's 2005 reconstruction of other trade routes, we see trade networks up to 10 times that distance. These distances were just too long for nomadic people groups to be so consistently leaving items. The movement of obsidian is so consistent, that if it were to reflect simply a movement of people, you would have a one-way flow of people from sources to destinations.

In addition, one can ascertain origins of the rest of the artifacts you find at a site, and if certain types of materials are consistently coming from farther away, these are more likely acquired by trade rather than manufactured.

In addition, there has been some ethnographic research on the ranges in which hunter-gatherer groups operate, which appears to scale, with the largest figure I have seen indicates a range of 32 square km for a group of 100 hunter-gatherers (which would be an extremely large group), which indicates that distances over 100 km are significant evidence of trade between people groups.

As for preferences, as far as I am aware, the preference seems to have been practical things, such as raw materials for making better tools.

It would be extremely difficult to ascertain exchange rates and methods without some sort of written record of the transaction, which by definition can't happen in prehistory. So unfortunately I can't really help you there. We can sort of ascertain relative value of certain items (i.e. item x is more valuable than item y) based on distribution within sites and the distance the commodities traveled, basically an obsidian tool would have been more valuable than a chert tool because obsidian was harder to get and the obsidian was traded over a longer distance than the chert. But I can't see how we would be able to quantify exactly how much more it was worth.

One last thing to note, there are two kinds of trade, direct trade where one vendor travels around and trades his wares directly with everyone, and down-the-line trade, where a vendor trades with one place, and that place trades some of the stuff they got from him to another place, and that place trades some of that stuff to yet another place and so on. The latter system is most likely what we are seeing in the record.