Plan the full cost of a kitten in its first year.
A kitten's first year is dominated by veterinary work rather than equipment: the vaccination course and the spay or neuter together usually exceed the adoption fee, the carrier, the tray and the scratching post combined. The ongoing side is unusual in that litter rivals food as a recurring expense, which is why it is broken out separately here — it is the line most people forget entirely, and at twenty a month it adds up to a meaningful share of the annual total.
Kitten first-year budget
First year = (adoption fee + vet and spay + gear) + (monthly food + monthly litter) x 12
Spaying is abdominal surgery requiring a longer anaesthetic and more recovery monitoring; neutering is not. Expect a female cat's procedure to cost noticeably more, and enter whichever applies.
Clumping clay litter is cheapest per month but heaviest to buy; wood pellet and paper litters last longer per bag and often work out similar overall. The larger saving comes from a covered tray, which cuts waste from scatter.
Less in trauma and parasite treatment, more in enrichment and often in weight-related veterinary care. Over a first year the difference is small; over a lifetime indoor cats are cheaper mainly because they live longer.