Well for me, it would be:
$470 for the console
$120 for an extra pair of Joy-Con
$40 if I want a Joy-Con charging station
up to $100 per game (MK8DLX is $80, Zelda BotW is $90, Mario and Splatoon 2 are $100, Has Been Heroes is only $40)
For comparison, a Wii U with 4 games (MK8, Nintendo Land, Sports Connection and an eShop code for SSB) is currently $430, Wiimotes are about $70 each, a Pro controller also about $70, and games are around $70 as well (including BotW). And that's taking into account the fact that this is at the end of the console's life cycle, and I think about the only thing that's changed significantly since launch prices is that you get more games with the Wii U itself. (Google agrees - launch price for the Wii U Deluxe was $430 with nothing included.)
But also, to factor in, a New 3DS XL is $250, with $60 being fairly typical for a top-line new release game. I can't imagine that the Switch will really replace the 3DS as the portable console, but it might eat a little into the time I would otherwise spend on it and hence the games I play on it.
The conversion to GBP is about 2/3rds at the moment - so from some quick calculations the costs aren't that much off what you're talking about, I guess we just have differing ideas of what all that stuff is worth.
ARMS is a tech demo, and one with fairly limited replay potential.
Eh, I dunno. I think it's got a bit of replayability. It feels like it's probably on par with stuff like Pokken, but I doubt it's going to be a smash hit like Splatoon was.
I think it's a bit below Pokken, personally. Besides the lack of a major licence, I didn't feel like there was a huge amount of control available like there is in most popular fighting or shooting games - you pick a character, you pick their left and right weapons, you flail wildly in an attempt to hit your opponent without getting hit yourself. It didn't feel like they were leaving room for, say, fancy combos like in Street Fighter or a large range of weapons like in Splatoon. I guess I might be judging too much from a pre-launch demo, but none of our group really saw it as a game that we'd pay full price for.