I suspect this taboo topic of money is related to the guilt attached to a lot of games purchases. Another miniature or pack of magic cards is a self indulgence in a lot of cases, making the issue of spending a lot a reflection on the character of the customer. Additionally, the mental category of "optional purchase" brings out the cheapskate in most people, because although we can emotionally write off fuel and food and shelter, and perhaps even a night out as a social cost, there is little to no emotional leverage to justify buying what is essentially another toy.
In some ways this guilt is unfounded, because there is often a large amount of social, entertainment and self actualization value in these products that far outstrips other spending you might do - e.g. hours spent painting and playing with friends and family a board game with miniatures in it.
This emotional backdrop sees us regularly hand over massive amounts of money to parasitic supermarket cartels without any thought, yet mentally challenge the price of every game purchase made. Thus the taboo.