£600+ for side skirts is bit steep especially if it dont include VAT & sprray+fiting!
if u mean would the car be worth more(with styling kit) when it comes to selling it then shouldnt think so unless is standard factory fitment which i dont think it is on a lexus is200. is like 'max power'(sorry!) feature cars that got full body kit, lowered, sound install, flip paint etc.......when these owners come to selling it they put the price up sky high(higher than what its actually worth) because they spent say 10 grand on modifications not to mention time & effort. i always had impression from friends/family that however much you spend on the car in terms of mods it wont increase the for sale price(some people do increase it anyway), if anything it de value the car's price.
when i looked at modified( or slightly) cars i always had salesman saying well its lowered, got new alloys etc when i try reduce the price!
my experience / thoughts anyway.