In our group, the GM bought the first set and introduced us all to this particular system (he had gotten us interested in a couple other free demo rpgs before), but other people in the group have been helping to buy the expansions. Especially when our GM says he'll add in any rules from an expansion we get, it gives us incentive to help pitch in and buy some. So far our GM bought the starter kit and Gathering Storm, and rest of the players have pitched in to get the Adventurer's toolkit and Winds of Magic. With 6 people in the group (I know, it's a lot, but usually only 4 or so show up at any given event), we each just pitch in $5-10, depending on how much the specific expansion means to us (our magician players really wanted the other orders and expanded magic rules in Winds of Magic). This has worked so far, as it allows us to add more rules without our GM having to spend tons of money (he pitches in at least $10 per expansion, but we usually end up paying the rest ourselves).