I think the challenger decks are a pretty good baseline for this... reasonably synergistic, fun to play and fit a theme more than the cheap starter decks. They're 30-ish. You could round it out with better cards and easily have a fun and decently competitive tabletop deck for $40-50. For Modern, that $100 mark is for reasonably competitive decks. I've got a modern homebrew that's like $60 on paper now (i think it was $25 when I built it) that won't win any tournaments, but it wins a lot of games. (Maybe .500 win rate, but has beaten a lot of expensive decks it shouldn't have and that makes it worth it :p) 8-whack prices have come down, so it looks like you could build that deck, (even with
Goblin Guide in it) now for $75! (even the "budget version" MTGGoldfish did back in the day didn't have those and is crazy competitive!)
As far as Pauper, many of the tier decks are $50, so might as well go big! hahaha. The problem with going super cheap is that there are so many Pauper staples that are like $3 each that really do make a big difference. (
lightning bolt,
mutagenic growth etc.) You can build a decent decks for $25, but when you can get really strong decks for 2x that, it's hard to argue :p