Why one must to add talls or tallet?
because it means something else without it.
Det koster tusen kroner: = it costs exactly 1000 kroner.
Det koster tusentalls kroner`= it probably costs more than 1000 kroner but less than 10,000 kroner
It is hard to me beacuse I dont speak english but I do my best.
What's the difference between: tusentalls and tusenvis av?
They can be used interchangeably, but "tusenvis av" sees the most use.