Personally varies from month to month, some weeks purchase brand new cars, other weeks (especially when budgeting) specifically purchase second hand (cheap) cars or accessories.
On average 1-2 cars a week, however this amount often changes and usually dependednt on the time of year, swapmeets, holiday periods etc.
I think a strong indication of the level of interest can be measured by the amount of time spent carrying out any slotcar related stuff. Whether browsing, racing, reading & modelling etc - not just the amount of purchases alone.
The hobby is expensive & addictive collecting the latest products alone, it's best to avoid the classic stuff...