GW has been raking in massive profits and inflates their prices every year. Underworlds is basically a competitive card game, and so keeping up with it even at the best of times can be stupid expensive, unless your group is really casual.
I've heard MTG players on average spend more money than Warhammer players, which sounds wrong to me but I don't know enough about MTG to dispute it.
Regardless, I wouldn't suggest Underworlds if you're looking for a cheap Warhammer alternative. I'd be hesitant to even suggest something like Warcry or Necromunda, because I can only assume GW will continue to inflate prices, though obviously it's still a good starting point.
MTG prices depend greatly on the format you play. Like for example, in Legacy, a 4k deck is considered on the cheaper side, while modern decks go for like 500-2k. ---
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