I do a single exchange of a single karma for cash (and vice versa) per adventure/mission. Players can exchange karma between themselves for a similar fee (5k nuyen minimum), but only for a single point per set of player exchange. So, if one mage wanted 5 karma extra from the adventure, he could buy one at the set rate (5k nuyen) from the GM, and could buy one from each of four (4) other players at the table, for a minimum fee of 5k nuyen each or 20k nuyen total.
This makes it more expensive than build points (since each is effectively worth 2 karma and can buy 5k nuyen, themselves). However, if the players need a bit more cash, then it works in their favor. I've always felt that money is a bit easier to come by, but then, I'm the one of the ones who began selling taxi drivers as meat to the local street doc. (Get a taxi ride to a street doc, shoot the driver in the face, take body inside to doc, walk out with money, what could be easier?)