Cycling is becoming increasingly expensive and a lot of it is purely marketing and gimmicks. However, here I will recommended some products I do think are worth spending money on.
Shoes: Good shoes make a massive difference and should be a priority. I recommend Lake shoes. The 238 is great for wide feet and the 332 and 403 are really good performance shoes. Bont work well for those who like a Flat shoe. Sidi are well made but a little narrow.
Insoles: Hugely important. Brands I have used so far and recommend are G8 and Sidas 3 feet slim.
Saddles: Selle SMP and SQ labs
Pedals: Shimano SPD SL pedals 105 and upwards are the best 3 bolt pedal. The Ultegra +4 mm pedals really suit wider riders (roughly over 75 kg). Look can suit smaller riders. Speedplay have gone downhill with the Wahoo takeover, they also don't suit big footed powerful riders as much.
Short cranks: Lots of people can benefit from shorter cranks.
Narrow bars: Most bikes come with bars that are too wide stock.